I use the M1a2 (I believe it's called?) Grenade Projector, and just pop a tennis ball in there. SOmetimes I have to wrap it with some duct tape to make a snug fit....but....it works.
For launching tennis balls, normal 7.72x39mm blanks will work, but if you wish to launch an honest practice grenade off, you need to find full house grenade launching blanks-they're rare as hen's teeth, though some folks I know reload them, I've seen the load data on some other site, I believe the FALFILES might have it somewhere.
To launch a grenade:
*push in the gas lock, and rotate it upwards.
*raise your grenade launching sight. This sight indexes off of a sight found on the original Yugo rifle grenade itself, or you can "guesstimate" on the end of the grenade.
*fit rifle grenade or projector on the end of the muzzle until it locks into place
*load gun with blanks
*if you're the incredible hulk, you can shoot them from the shoulder with the full grenade launching blanks-would not recommend it. Most people launch them by angling the rifle, this is how I've seen military pics shown doing it.
Note that you MUST use blanks unless you have by some miracle obtained an FN "Bullet-Thru" grenade type, and given how much you will pay for one, you will KNOW if you have one-those are able to use normal cartridges to discharge them. I've never seen one in private hands personally.
I hope that helps!