The format of the entry looks like this:
minute hour day-of-month month-of-year day-of-week command
minute 00 through 59
hour 00 through 23 (midnight is 00)
day-of-month 01 through 31
month-of-year 01 through 12
day-of-week 01 through 07 (Monday is 01, Sunday is 07)
All five fields must be specified, but an asterisk can be used to match any time/date field. So to build an entry that would run "command" every fifteen minutes you would create a file with a text editor and place an entry in it like this:
15,30,45,59 * * * * command
Which would run "command" at 15 min, 30 min, 45 min, and 59 min, of every hour, of every day, of every month, of every week.