Plug your new thermistor and heater core connectors into the Printrbot. They should fit perfectly. You can just set the hot end assembly on the print bed for now. We aren't going to do anything too taxing. If the wires are too short for that you probably did something wrong.
Turn It On
Cross your fingers, here is the moment of truth. Power up the printer and connect to it with your printer software of choice. I use OctoPrint but you can use whatever you like. First look to see that you can connect to your printer. Some software packages will complain if they don't detect your hotend or thermistor correctly. It should read about 20˚ because that's normal. If you don't see that you'll need to try and trouble shoot things. You are on your own.
Heat Things Up
Cross your fingers again and hold your breath. Things are getting serious. Tell your hotend to heat up to 50˚ and keep one finger on your hotend and one hand on the power plug and both eyes on your temperature reading. You should feel the hotend start to heat up as the temperature rises on your screen. You've passed the first test. Watch your screen and notice that it'll heat up to around 60˚ before it starts to taper off. You can now use the same software to tell your hotend to stop heating and if you have these kind of things graphed you'll notice something like this graph.
You can breath again. Everything worked perfectly the first time (if it didn't keep trying). Your hotend should heat up on request and your thermistor should recognize a change in temperature. Nothing will be perfect at this point, but you are making progress. Now it's time to rig up your extra fan.