The art of taking a car that has been written off by the insurance companies (deemed as uneconomical to repair) then putting time and money to prove the insurance companies wrong. We say art deliberately as you should not assume that all salvage projects can be repaired cheaper than just saving up and buying that car from a normal auction yard.
The first thing you need to know is not everyone who buys salvage wants to repair the car. So supposing you are buying a vehicle though a broker like SalvageSpareFinder, you may find yourself bidding against a breaker yard who sees more value in the car as parts than as a working vehicle.
So you should cost all your parts first, then add on a bit more, then take into account your time and labour, research how much the vehicle is worth when repaired and only then set a price as a limit, before entering any salvage auction.
If you do the work, you can end up with something that will turn a profit or save money, do it wrong and you could find a large bill and nothing more than a scrap car.