How do you put on a condom?

Me and my boyfriend have tried using condoms for a while now but they all currently haven't worked. Every time we put the condom on, his foreskin peels back all the way and doesn't move over like it should without a condom on. Also when we use condoms, his penis always becomes flaccid quite quickly. Does anyone have any tips to help us out because right now its really affecting our sex life.