how to customize register page

i would like to customize my register page.
I use Jonradio private site to make my site accessible for only registered member
and membership for controling register payement
i would like to know how to do to customize my login page
i have installed simpleModale login plugin to
but don't know how to configure those 3 plugins to work togheter
or maybe there is a better way for customising my login page
thanks a lot for answear
a kind of step by step would be great
best regards