Don't use tools steel since it hardens as you weld it, and the weld will most likely crack. Plain C1018 or 4130 would work great. I would recommend TIG method first, MIG second and stick as a last resort. You'll have to remove all the paint where you want to weld first. Make up you pieces you want welded to the frame, bolt your rearsets to the pieces and clamp the assembly to the frame where you want them to go, and make sure they're straight. Then TACK them in place with very tiny welds and verify they're where you want them. If you lay a weld bead on just one side without tacking all of the sides first, the part WILL MOVE! Small weld beads at first on all sides and then finish up the rest.
If you have a milling machine or know of anyone with one, see if you can get the side of your steel piece milled out with a radius cutter to match the frame tubing's outside diameter so the part fits around the tubing instead of it just being scabbed onto the frame.
Oh, if you have any electronics on the bike like electronic ignition, remove it before you start welding.