1. Figure out what time signature the song is written in. It's usually 4/4 but 6/8 is pretty common too. Sometimes you find gems at 7/4 or something weird.
2. Find a part that is good for looping. This is usually the verse or the chorus, and not very often the intro or the solo.
3. Now, find out exactly where that part begins. Do that by zooming in and looking at the all the peaks in your audio editor. The snare drum peaks are usually a bit larger and longer than the bass drum peaks.
As long as you navigate using the peaks looping will be a lot easier.4. When you've found a good beginning, you have to find an end, too. Here is where you need to think of the timescale. You need to find a point in the song at a multiple of the number of beats in a bar. For a 4/4 loop you would usually look after 4 (one bar), 8 (two bars), 16 (4 bars), 32 (8 bars, sometimes the whole verse/chorus) or 64 beats (you could often fit a whole verse and a chorus in this). When you've found a good length cut just at the beginning the next bar. This is usually at a peak, too, unless there's some weird syncopation going on.
6. Listen to it looping for a while. Is the loop "tight"? If not, zoom in a bit and adjust it by cutting out small pieces, one at a time. Does the loop sound natural? Did you accidently cut it in the middle of a singing verse or something? Try again!
Simple 4/4 2 bar example:
Not so simple 6/8 1 bar example:
