Step 1: Materials
Preferably the syringe should be large and have a scale with divisions to sixty.
Step 2: Mark Scale
A 60ml syringe is most suitable for making a logarithmic scale. The scale is practically readable to depths of 15-20m which I think is still comfortable. More extreme freedivers going deeper might want a more advanced instrument.
The depth marks should be as follows:
0m = 60ml
5m = 40ml
10m = 30ml
15m = 24ml
20m = 20ml
30m = 15ml
40m = 12ml
50m = 10ml
172m = 3.3ml (World record 2005/10, totally insane!)
Step 3: Make a Hole at the Surface Mark
Step 4: Seal the Tip of the Syringe
Step 5: Go Diving!
Before you dive you insert the piston on and the syringe will contain 60ml of air (at surface pressure). When you dive and go deeper the air inside the syringe is compressed and the piston moves to equalize the pressure difference inside and outside the syringe. When you return to the surface the air in the depth gauge will expand and and the piston moves back.