Well, obviously, first you need a condom. Preferably, not one with lubricant. Flavored condoms are usually best, but unlubricated are fine too (although I'd recommend a little flavored lube for the unlubricated ones).
The next step, is to unwrap the condom, and put it in your mouth. Place it on the inside of your cheek, so that the resevoir tip is pointing inward, between your teeth. DO NOT BITE! With practice, you can carry on a skillful conversation like this. Below, we have a cross section (poorly drawn, by yours truly) of what it should sort of look like.
The final step is to work the condom over to the front of your mouth with your tongue (as little teeth as possible, please!) Once the condom is at the front of your mouth, put your mouth on the penis (or banana as the case may be), and begin rolling it down with your lips, tongue, and teeth. (Yes, gentlemen, as LITTLE teeth as possible, even if you know what your doing. Tears are bad.)
So that's it! With practice, you can be a pro, like my purple banana fetish friend here. (have you ever seen a worse .JPG? scary.) Don't forget, whenever you're applying a condom, be it orally, manually, to a penis, banana, dildo, whatever, hold the tip as you do so, to get rid of all the excess air. Excess air causes rips.