You can fix those easily enough with a bit of welding equipment, but since you are asking how, I wouldn't. A novice welder is far more likely to warp the surrounding sheetmetal than to get it right, but there are many options:
First, you need some patch material. Ideally you go to a local wrecking yard that will let you use a battery-powered cutoff grinder or sawzall and harvest some pieces that have the correct curves and cut them to a good size. If you can't make that happen, perhaps you can get them to cut out a chunk for you. Starting with new metal is problematic since auto-body steel is treated and formulated to form with less tearing than plain steel you would buy at the store. Once you have your patches of about the right shape and size made up, then you can decide how best to attach them.
There are some lovely modern adhesives that will bond strongly enough to be considered permanent, just enlarge the hole a bit and slide the patch in from behind or pound it in a bit and attach the patch from the outside.
If you really want them welded in, use a high-speed cutoff wheel in a grinder to cut the holes in the cab back to good (at least 70% of original thickness) material, cut your patches to fit precisely, and take it to a fabrication shop to have them do the final welding. Shouldn't be more than a couple hundred bucks that way.
If you really insist on doing the welding yourself, buy a car door to practice on and a little wirefeed like a Lincoln weldpack-100.
You have a better chance of success if you use one with shielding gas, so you will need a small bottle of argon/CO2 mix and some fine (.023) wire. Once you can weld a patch on a steel door without warping it too badly you are ready to attempt it on your cab.
Remember that if you weld in a patch you have to make sure to get some sort of corrosion protection on the backside of the welded panel or it will rot out again.
good luck,