There are different ways to hold your child for baby pottying, e.g. the newborn hold; see this picture here of me holding my then 4-5 week old, or watch the video below:
Holding over the toilet is great when you get the hang of it – no cleaning up! Also good for bad backs because you can rest on the toilet seat yourself. Heres me with my then 11 week old:
A deep squatting position helps relax the pelvic floor and makes elimination easier. When they are older you can use books etc to help them stay put if necessary. Sometimes, if an older child is just starting potty learning, you may need to wait until he or she is actually starting to eliminate before physically placing on the potty and reinforcing the action with your words and noise.
Once your baby can sit up, they are strong enough to rest on the potty themselves. You will know when this feels OK.