There are 2 ways to do this:
1. Wire each individual piece with a plug and then plug them all into a power strip and then plug the power strip into the thermostat (this is the simplest method and generally is less likely to fail than #2)
2. Wire the first piece with a plug, then add on each additional piece with regular wire.
I don't like doing either of these as there are a lot of electrical connections that can fail. These methods also leave multiple heat sources without a thermostat directly monitoring them. Yes they are all being controlled by the thermostat but it is still possible for one of the pieces of heat tape without the thermostat probe attached to it to short out and overheat/cool off/start a fire. This is why I only use single strips of heat tape. 1 heat source per thermostat probe. Some people might think I am being overcautious, but to me it is worth the extra cost.