Lay on your back and put his tummy on your chest or your tummy, so you will be chest to chest. Try to encourage him to lift his head by talking to him and giving encouraging words.  If he is very young and this is his first exposure to tummy time, you can pick him up a few minutes (or even seconds) into fussing.  Then increase your goal to 5min next time, then 7min, 10min, 12min, and 15min.  Your goal is to get to at least 20min of tummy time 4 times a day or 25min of tummy time 3 times a day.