Well, I can see some improvement in the welds between the d-ring mounts and the new mounting plates. That being said the welds on the d-ring tabs don't look very good. Of course that's going off of the dark cell phone pics...
The mounting plates bring up a different issue. No matter how strong the rest of the bumper is, you just created a weak point. By cutting them off straight and just butt welding in the new ones, those joints are weaker than the rest of the mounting plates. This is from both the joint itself, and the heating (and stiffening) of the material. Knowing where that one forward mounting bolt is to the frame rail, that leaves the new joint at a somewhat flexible area. Any angled pulls from straps or the winch, bumps to the corners of the bumper, etc will be putting stress on the mounts in a side to side direction, essentially trying to fold the mounts. That weakened area will speed up any chance of it folding/tearing. I'd look at adding some reinforcement down the side of the plates and if possible adding gussets to the outsides.
For the welding itself, best suggestion I can give you is to look into welding courses. They should go over quite a few things that will help, like equipment setup and usage, material prep, proper techniques, what to look for in a weld, etc. Besides that it comes down to experience. Take any chance you can to practice.
