Quickest method is to head to any ship or army and engage them. When given the option to continue the attack, surrender and wait until they sell you to slavery.
Choose either to escape alone, with help of other prisoners (which will take longer since you need to amass all prisoners to follow through with your plan) or contact help from the outside (only possible if you are the leader of a faction).
Once on the field, head straight for the edges, keeping a distance from any guards in the area until you have escaped. Once escaped, go to any enemy and get recaptured and sent into slavery again.