India is a large target and can be difficult to conquer if approached incorrectly. These guides highlight a couple of useful and well tested strategies.
If you are a European Power
With In Nomine, India is not quite so wealthy as before. The huge population will not benefit income until late in the game (if ever) or unless there is a land connection between your capital and Indian provinces, and the tropical modifier will greatly decrease the income of all the inland provinces. Nevertheless, India is still rich and easy to conquer, and by taking any one of the many coastal provinces it is possible to enact the powerful Form East India Company decision (Found the VOC for the Dutch), making it an attractive destination for any would-be emperor.
Timing is key for the conquest, and will drive your planning. Too early, and no European state is rich enough, technologically advanced enough, or well-positioned enough to easily compete with Indian states. Too late, and it is impossible to take advantage of greatly lowered badboy for conquering heathen provinces. The best time to start is the late 1500s or early 1600s, by which time gunpowder units are becoming very powerful, it is possible to amass a very large technologically advanced army in comparison to any Indian power, and it is relatively easy to conquer or colonize staging areas.
Before this point is reached, it is necessary and useful to develop staging areas. The Maldives, Sri Lanka, the eastern coast of Africa, the Indian Ocean islands, and South Africa are all potential places to base naval and land forces out of in preparation for the main conquest. Unless playing as Russia, the Ottomans, or some other land power that has actually conquered their way to India's borders, it is probably easiest to focus first on whichever area of the country is divided into many small states and is far away from the major regional land powers, usually the southern area of the subcontinent. As with anywhere else, looking at the webs of alliances and guarantees in place is very useful in ensuring that only a limited war is fought. The technological backwardness of most of the states here means that infantry/artillery stacks can be used as the main fighting force. 10-15000 man stacks are recommended as it is possible with those to easily assault any sieged provinces immediately and capture them thereby reducing attrition significantly, which otherwise in the tropical interior will be severe. Otherwise, if cavalry stacks used strictly to attack enemy armies are preferred, they can be divided into groups of 3000-4000. With a good technological lead, this strategy should be enough to defeat Indian forces 3 or more times greater.
Now that staging areas and a target have been acquired, the process of conquest is relatively straightforward. Transport several stacks from the homeland and a small-medium size fleet to the staging areas, then declare war on the target. Invade while using the fleet to cover the transports and blockade the enemy's ports. Try to grab as much land as possible while keeping a weather eye on BB. Once that state is defeated, declare war on another or fight against the allies or guarantors of the first state. Essentially, the strategy here is to continuously attack and conquer Indian states while waiting for truces to expire. Unless BB-wars are part of the player's strategy, India happens to be the last area conquered as part of a world-conquest, or the player is simply extremely powerful, it may be necessary to temporarily pause to wait for BB to drain, though the much lower BB cost of conquering heathen provinces should alleviate this somewhat. Once all this is done the player will be ruler of India, congratulations! Make sure to take the "Form East India Company" decision and take over the centers of trade just conquered.
Pre In Nomine
Alright, so you're looking to conquer India? Good choice. It has many valuable provinces, COTs; Not to mention it's smack dab in the middle of Asia, and therefore a portal to later conquests. But let's not be too hasty. You're going to need Deus Vult as a national idea, and you probably won't have the tech, manpower or money to conquer India straight from the get go.Build colonies in S. America, the Caribbean and S. Africa before tackling India. Wait until the 1700, when controlling Brazil, the Greater Antilles and several provinces in South Africa.
Now that you have your colonies, build up a fleet and put an army of 7-12 units of infantry on it. Sail for the Maldives, DOW it and annex it as soon as you can, it's an island, and its allies, if any, probably won't bother to send troops your way. Then build a regimental camp there, and wait a year. After the regimental camp is done, get your army on the Maldives on a ship to Sri Lanka to conquer Ceylon. Same deal, annex it as soon as possible. and build a regimental camp in both provinces. When this is done you have a sufficient platform to attack India. (Personally, though, i would conquer the area around Bengal as well.)
First thing, you're going to want to build up as much infantry as you can in the Maldives and Sri Lanka, (and Bengal if you conquered that as well.) Then attack the power controlling the southern tip of India. Send the two Units from Sri Lanka to the closest to provinces and capture then, land the troops from the Maldives in the province of your choice, although i would stick to the tip of India. When you have captured all three of these and withstood their measly attacks for a while, you can take a couple of those provinces from them.
Wait for the truce to end and DOW on them again and again, until you've annexed them completely. By now you control the southern part of India, and conquering the North shouldn't be that hard by now, especially if you're conquered Bengal. Now that you have India, Asia is waiting for conquest.
NOTE: Your conquest of India will give you a LOT of BB. One option is to wait until your BB is sufficiently low until conquering again. The more favorable option would to be such a powerful country that you don't need to worry about BB. For this i would recommend either France or Germany.
As the Timurid Empire
The Timurid Empire is in its own league when it comes to conquering India. It's in a perfect position. All it needs is the troops it already possesses, a little more money, and strategy. Let's start with the first step, getting more money. First off, stop researching your naval technology and eliminate the fleet, you won't need it. Next, diplomatically or militarily vassalize Chagatai and Kazakhstan, they won't give you very much, but they'll give you enough. You will also want to send your merchants to your two COT's, get some revenue from them. The next thing you'll need will require you to start your conquest of India; conquer Sind.
It doesn't matter who Sind is allied with: Sind MUST be conquered, or you conquest of India won't go quite as smoothly as you'd like. But there is no need to worry about it, Sind is only three provinces, the conquest should be easy and should only take two wars. In the first war take the COT Kutch and the other province that Sind owns, in the second, annex them. When you have taken over Kutch, your income should rise sharply, and continue to do so. Keep sending merchants to Kutch, and never leave it unattended, it's your moneymaker for the moment, and it will become a very important COT as you expand into India. In my game, Kutch reached a value of about 2,000 ducats.
The next step, vassalize all of the Muslim nations in India by any means necessary. Just do it, it will be very, very helpful. And it will speed your progress into India significantly to diplo-annex them one-by-one. Then, when you have all of the Muslim nations either annexed or at least vassalized, attack the Hindu nations, and annex them as fast as you can without getting into a BB war.
As you are expanding in India, your military and technology will far surpass any nation in Asia. You will rival the Europeans. If it pleases you, you can westernize and pass them completely, becoming the dominant force in the world. You could even colonize the world just as the Europeans do. You could create the Mughal Empire (though that would make you be in the Indian tech group, which is not a good idea if you plan to be advanced. In a twist of irony, you could colonize Europe itself. (Imagine Indians ruling the British) It may take a century or two, but conquering India will enable you to do whatever you want.