This is too complicated to really well answer online, but I will tell you generally what I do. I make iron mounts for 19th century southern guns so I am making basically the same butt piece and guard. I make butt pieces from two pieces of 1/8"-3/16" thick weldable steel sheet that is cut, forged and forge brazed after riveting.
I make most guards out of two pieces. One is the bow and front extension. The other is the grip rail and rear extension. For the grip rail/rear extension, I fold over and forge weld at the spur. I used to rivet the two pieces together with brass shim stock and forge braze. Now, I forge weld the riveted bow/grip rail connection.
When brazing, I used 1/2" square mild steel for the bow/front extension and 1/2"x1/8"-3/16" mild steel for the grip rail/rear extension. For current, totally welded, guards, I use a single piece of 1/2" square x 8" long wrought iron fence pickets.
East TN guards are made of three pieces that are forged, riveted together and welded. The rear extension and grip rail is riveted and welded flat and then bent to shape. That is then riveted to the bow and welded, I have a special anvil made of folded 1/4 plate about 12" in front of the port on my propane forge that I use for the guard welding. It is the only way you can keep the iron hot enough long enough. The next time I do it, I am going to use two rivets between the grip rail and bow so that it doesn't twist.