I agree with onboost, Celettes can be tricky to use, the really difficult part is knowing how pull on the 911, you have to know the car inside and out. You can do a lot of damage if you're not delicate. It's not like some old pickup truck frame you can anchor into and yank, the 911 is just sheet metal, it's tough to get a grip on it and it can be easily pierced or torn.
You have to fabricate custom "pulling fixtures" to get a grip. You also have to make a lot of different "bearing surface fixtures" to distribute the power of a pull and spread out the force over a large surface area.
The video below is great, shows the process, it's a factory Celette training/demonstration video. At the end, the guy smears seam sealer all over it, it looks terrible, kind of cracks me up. Overall great repair though.
https://www.youtube.com/watch?v=42UKWU88b7U
Also, the Wielander & Schill Vario Drill WS90 is the best tool in the world for taking these cars apart. Depth set drill, only goes though one layer of sheetmetal, very precisely, leaving the good stuff behind unharmed. He uses one in the above video and here's a few below of it.
https://www.youtube.com/watch?v=pWGWL6mBZ-Y
https://www.youtube.com/watch?v=9VVniLdF2u0
Below is another great way for breaking spot welds, the belt sander.
https://www.youtube.com/watch?v=hvHiJQL7_9s