Cryomundus gave a nice account of how to do this. I just supply some images to go along with it.
Done with
Slade-3.1.0.1-626-g0336db5
You can do this manually by setting the offset for each weapon sprite lump using the Offsets: bar on the bottom so that the weapon
sprite is centered as indicated, or use the Auto menu.
Or, you can highlight all weapon sprite lumps, right-click on the highlighted lumps and use the Modify Gfx Offsets menu
and select Hud/Weapon (Doom).

However, if you choose the automatic way you may find that the weapons are shifted a bit too much to the right.
This can be overcome by selecting Set Offsets and shifting the weapon sprites so that they are centered.
A bit of trial and error may be needed at first.
