Some advise:
1)Fitting - Fit a handle to the tang and rough shape it. It can be a piece of pine, or the actual desired wood, but you need something to do the fitting with. It will allow you to calculate the finger clearance and feel how the hand fits. The knuckle clearance should be just slightly larger than needed, but have a snug feel.
As suggested, paper and cardboard are your friend here. Cut a strip and put it on the tang. Put the handle on next. Now, grip the handle and wrap the cardboard around your knuckles, marking where the tang is on the butt side. punch a hole there and see if the fit works. Move the hole on the cardboard until the guard fits the hand. Next, make a test guard from thin sheet metal or aluminum. Fit it to the tang and bend it with your fingers to the desired shape. If this fits and looks good, make a wooden shaping block to fit the prototype, and shape the final guard on this block.
2) Bending - Take to heart what I said about the way a bar of steel will bend. Bend it while the width and thickness is even. Anneal and bend while soft. If possible, do initial bends at dull red heat. Take a look at what you got from this failed try, and see how the bend angles don't look so attractive.
3) Proportions - If the handle is going to be too large for the blade, a D-guard may not be a good choice. Usually the blade will need to be at least twice the handle length to support a D-guard. 10" and longer works best. 8" is about the minimum. The "trench knife" look of your design will be OK, but a larger blade will have a better balance and feel, as well as look. Also consider the mass of the handle and guard compared to the blade mass. This greatly affects how practical a knife would be for defense.
4) Welding - Use a piece of 1/4" threaded rod or bolt. Weld it solidly to the tang, keeping the rod centered to the rectangular tang. Once the weld is cooled, temper the blade to remove weld embrittlement. Then grind the threaded rod flat to match the tang thickness. The threads remaining on the sides will be all that is needed for the tang not or pommel.
Some practice will improve your welding, but if unsure, get someone else to do it. The tang weld is a structural part of the knife, and needs to be right. The rod also needs to be strong enough itself. A 24" piece of thread-all and a 24" piece of mild steel from HD will cut about six practice welds. When possible, make a slot in the tang, and grind the rod to fit the slot snugly. A few hammer blows will expand the rod to a tight fit. Then weld the rod around the slot on both sides.