There are two types of welding commonly used on vehicles, namely: 

• Oxy/Acetelene (Gas) welding where the surface is heated with a flame and steel filler rod is hand fed into the hot spot.
• Arc / MIG Welding where the surface is heated with an electric current

Gas welding is very controllable and is used for careful work with thinner metals
Arc welding is more common in the DIY market but it does not give as neat a weld as a MIG welder.

It is upto the welder to decide which welder to use.

When welding larger flat sections it is important to tack weld the plate on first, this is done to prevent the plate from warping from the heat, here an electric welder is the best to use as the GAS welder tends to heat the area too much.

Once the tacks are made (Each corner and half way between corners) you can weld it in completely.

Take EXTREME care when welding on a vehicle, check what is behind the welded area, check proximity of fuel lines, rags, electric cables in the vehicle and on the floor etc etc. When you are happily welding away, there could be a small fire burning somewhere which you don't smell or see and when the weld is complete you have a major emergency on your hands.

Further care must be taken when using an electric welder on a vehicle, common practice is to remove the battery terminal and alternator connection as some systems have bad earths, it is perfectly safe to connect the earth clamp DIRECTLY to the welded article so that the welding current travels directly to the weld, any other connection may route itself via your electrical system and damage sensitive items.