What is the safest way to choke a ual partner?

My playmate and I have escalated our dom/sub relationship a bit lately. We are trying to incorporate asphyxiation. I've read up pretty thoroughly on different chokes (air or blood, etc.). I will be attempting a blood choke to the carotids. My question is what is the best way to go about doing this. We have lightly practiced without and I'm having difficulty getting a good hold on the arteries. Any suggestions to help get to a quick "out" (within 10 to 20 seconds) without damaging the trach would be appreciated greatly. Thanks everyone.