Listen carefully to the music that you want to sample before you actually take pieces from it. You've already decided that you want to sample the music, so you have to find the best parts. Once you've familiarized yourself with the music, you'll know exactly how you'll loop the sample when you take it.
Extract the sample from the music. (Extracting is also known as "chopping.") Use a hardware or software sampler, such as an Akai MPC or Sound Forge to take the sample (there are many other devices or programs for performing this task).
Program the sample into a pattern and loop it. Again, the choice of sequencer -- hardware or software -- is yours to make. If the sample has original percussion in it, decide whether you want to add to the percussion or cover it up with your own snares. You want your beat to make sense in the end, so you don't want snares in the sample sounding wrong and awkwardly placed. (You can also use kicks and other pecussion of your programming to cover original percussion, if you deem it necessary.)
Place snares on the backbeat of the loop. In a 4/4 pattern, the second and fourth beats are where the snares typically go. Play the loop and listen to it. If there are no odd percussive sounds in the sample, you can leave the beat the way it is. If you do hear percussion in odd places, you can begin experimenting with ways to cover it.
Add snares in places near where the original percussion shows up. Snare rolls work perfectly, because they take up more space and sound awesome if you master rolling a snare. Once you have the snares where you want them, you can use settings and effects, such as an equalizer, a compressor, reverb or a phaser, to tweak the sound and make it your own.