Check that your condom isn't past its use-by date. Squeeze the wrapper and make sure it’s still intact. Be careful how you take the condom out of its foil wrapper and not to tear the condom on sharp fingernails or jewellery.
Before you put the condom on pinch the teat or closed end, check that it can easily roll down your fingers to ensure that it is the right way round.
Holding the teat or closed end, roll the condom right down to the base of the cock. Put lots of water based or silicon based lube over the outside of the condom.
Finally, practice makes perfect. Don't be afraid to practice putting on condoms – on your own or with someone else – until you feel confident.