1) Set your toolbelt up in the way you want it in Wurm. Note which tool is in which slot (numbered from the left, starting with 1). Quit the game.
2) Open the file "~/wurm/players/<wurm playername>/playerdata.txt". This contains a number of lines that look like " toolSlot<X>=<number>". Remember the order from step 1, write down which <number> corresponds to which tool.
3) Create a new file "~/wurm/configs/default/blacksmithing.txt" (or any other name you want in that directory). Insert as many lines for setting the toolbelt (and custom keybindings and whatever else you want) as follows: