C.F.
The other posters have given a lot of good advice, but I have a few extra thoughts for you. When my girl was learning how to wipe properly, I put my hand behind hers when she went poo, so as to help guide her, etc. I also tell her every time she poos that she needs to wipe her bottom until the toilet paper doesn't have brown on it anymore. Sometimes she'll wipe just fine, but if she only does it once or twice, there's still some poo left behind, kwim? So I try to stress that we wipe until there's no brown on the TP because otherwise she'll have an itchy bottom!
Hope that helps!