I don't know what fitting info comes with the commercially available damper sets , but there is a bit of extra information about the fitting of the last set of homemade damper rods I used:
I used 8mm studding as this is easier to get and you don't need to thread a piece of rod.
If an adaptor is fitted to the top of the stanchion (early teleforks) this will need to be removed to fit the damper head/rod assembly, but can be refitted once it is in place. If the tie rod drops into the stanchion (and it will!), just use a bit of tubing to push over the exposed end of the studding to pull it up out, so it can be screwed into the top nut (use locknuts, and if possible locktite (oil resistant type if possible)).
Ensure there is a little clearance between the alloy damper head and the stanchion inner, or it will be difficult to fill the forks with oil.
Ensure the length of rod is short enough to not hit the oil cutoff spike at the bottom of the slider on full compression (so as not to bend the tie rod), and is just above the upper oil bleed hole in the stanchion (so as not to obstruct oil flow). You will need to check these points with the forks partially dismanted, but with the top tie rod mount in the correct place and fully screwed home.
If filling the forks with oil when they are in place on the bike, add the oil a little at a time, possible gently working the tie rod up and down to ease oil past the damper head. I filled the forks such that the damper head was just fully immersed in oil when the forks are fully extended, and used a basic 20/50 (or 30 if available), whether a dedicated fork oil is better I don't know, although I have heard it said that they swell the oil seals, and may be over-engineered for 40/50's forks.