I doubt I went about it the right way with Lucy, but I just put her on her back one night in my lap and let her immediately grab my finger with her feet and picked her up.
Started doing this at least a couple of times a day for the last 5 weeks and progressively did it more each day. Now she just rolls down my stomach and gets on her back and waits for me to tickle her. Its taken time, but she actually likes it now and its a big game for us.