Wait until she is slurping some solids like about 4-6 months.  Able to hold her head up a bit and not have a spine be so c-shaped.  I used a teeny potty seat on the floor and held her there.  If I saw her make "the face" I rushed her to the potty and sat her down.  I really think it is less effort for them to go on the potty.  They are using gravity and not straning agianst a diaper pressed against their bum and sitting on a hard surface.  Can you imagine pooping like that while strapped in a car seat?  I cant beleive I made my oldest do that once.