try to level up the drive forms as you get them, at least enough to get the first level of their movement ability, which will then become an ability you can equip when you get it.
If you wanna power level them, just be aware that areas where characters from your party are just walking around, able to be talked to, reset the drive you're on so you get all your drive guage back, so you can then go back out and transform immediately. It'll make leveling the first three particularly fast in the right places.
You can also get this drive-guage refund from exiting a world from a savepoint while in a Drive Form, which is more likely gonna be how you do it since most of the "walkin' around points" are temporary or out of the way.
Leveling the Limit Form requires drive orbs, which you can get a lot of in the Land of Dragons, most noticeably the little clearing early on with the fireworks and carts, they'll drop drive orbs when beaten up in normal circumstances. Master is grinded by just running out the form guage, so every time it goes one number lower it levels up, meaning you can technically do the same thing, but most likely will wait until it goes low and then refund your guage. The first two are fairly obvious, and you'll probably find a good place to level them fairly quickly on your own as you get them.
Final Form isn't able to be gained until the last world, and even then after a random use of a form, and that's gonna be fairly obvious to level in the final world, or Twilight town by that point.
I hope this helps you out, I forgot about this and didn't realize how this could be a pain for others playing if they never touched the Drive Forms, and you only need to level them a little to get access to their stuff, but if you need advice to getting them maxed out, I hope this has been helpful!
Oh yeah, and make sure to set Donald and Goofy's healing through items and healing magic to "Emergencies" so they only use them when they are critically needed, very helpful and saves you items and donald's MP.
And try to be efficent with the new MP system, since Summons and Cure use the whole thing up, you can use normal spells until their names turn orange (that means when you use it, they'll empty the guage), and then use a summon or something so you get the most mileage out of your mp.