You may have heard that condoms are only about 80% effective, but the truth is that they can be at least as effective as the Pill or other such methods if used correctly. It's important to always lube up, that will make it less likely that the condom will break and create a more comfortable experience for everyone. Use a condom during vaginal, anal, and oral sex (flavored condoms can make the latter more palatable, literally and figuratively.)
So, here's how it goes:
1: Wait until the penis in question is fully erect. While we're on this subject, ladies should be sure to be at full mast as well, otherwise sex could be damaging to the vaginal walls.
2: Carefully remove the condom from its wrapper. You may be feeling rather passionate at this point, but be sure not to damage the condom with your teeth or nails or whatever else you might use.
3: Pinch the tip so that there's room for semen and the like.
4: Roll the condom over the penis, ensuring that there are no air bubbles.
It's important to store your condoms in a cool, dry place. I would recommend keeping one condom in your wallet, but using this one as a spare tire, as being in such an environment will reduce effectiveness.
And, ladies, use a dental dam (a condom cut in half or a piece of plastic wrap) when receiving oral sex-- this may not sound very sexy, but neither does Herpes, eh?