The most common way is to use the clip on the side of you reel spool. All you do is cast out, to your desired area, then put the line under the clip. This ensures that you cast the same distance each time.
However, if you are after large fish it may be best to do the following method. Put an elastic band onto your spool, at the base and quite tight. Then cast out, put approx 2" of line (from the spool, do not break the line.) under the band, and mark the line with a permanent marker pen. Then each time you cast you'll get the same distance. The difference being that when you hit into a big fish the line can pull free from the band. When the line has pulled free, all you do the next time you cast out is watch for the mark on your line, and then put the marked line back under the band. You will then be casting the same distance again.
It is easy to do Benny, it's just awkward for me to try to explain by writing it