If you run your bottle remotely (so it stands vertically), you won't need an anti-syphon, as the liquid will be kept at the bottom of the bottle by gravity alone. Of course if you're already on air, you don't need one either as it doesn't turn to liquid like CO2
I'd also recommend getting someone who knows what they're doing to fit it, although it's not particuarly difficult. They're basically just a short piece of pipe with a fitting which connects to the bottom of the CO2 valve.
But if you're fairly handy at working with CO2 bottles and do want to fit it yourself:
First screw your bottle to your gun, as you would have it when playing and use a pen to mark a line on the valve showing the uppermost point. Take the bottle off, empty it completely of gas and remove the valve from the bottle.
Attach the anti-syphon to the bottom of the valve and then bend it gently (being careful not to kink it), so that when the valve is refitted, the end of the tube will sit close to the inner wall of the bottle (the angle of the curve will be different depending on the size of bottle - 7/12/20oz) - this will probably take a couple of goes to get just right.
One the curve is sorted, remove the anti-syphon and use either PTFE tape or loctite (242/243 - not 271) on the thread and then refit - tighten it up so that the end (highest point) of the anti-syphon is exactly in line with the mark you placed on your valve earlier. Doing this means that when you reattach your bottle to your gun, the end of the tube will be sitting at the highest point of the inside of the bottle, drawing vapour, with the liquid lying underneath it.
Re-fit the valve to your bottle (again using either PTFE tape or Loctite) and tighten well - best to hold the valve in a vice and use a strap wrench on the bottle to torque it adequately.
Job done. Coupled with a decent expansion chamber, you shouldn't then draw any liquid into your marker.
One thing to note is that because all asa adaptors have slightly different thread lengths, an anti-syphon bottle which works on one marker, might not work on another as the highpoint may be different - in the worst case, the end of the tube might even end up pointing downwards and act as a syphon, drawing only liquid and no vapour!
EDIT - boring fact, but apparently both siphon and syphon are correct spellings, wierd.
