Some people apply framesaver (or linseed oil or something similar) to the frame as a rust preventative. If you do that, that comes first. If you use linseed oil, keep in mind that oil soaked rags have started many fires from spontaneous combustion, thus take appropriate precautions.

If you have the tools to install the headset race on the fork and the star nut, that also is a good first step. I suggest you do not cut the steerer tube right away, or if you decide to cut it, leave it a bit longer than you otherwise would want it. You can always push the star nut down a bit further and cut it again later (or have someone else do it for you). But if you cut too much, that is not an easy thing to undo without a time travel machine. On my last build I cut the steerer tube three times to make sure I did not accidentally cut too much the first time. It took more work to pull the fork off the frame to cut it again later, but it was worth it to make sure the final cut was where I wanted it in the end.

Once you have those things done, everything else comes pretty easily.

The last thing, I like to use strapping tape to tape my cables onto my drop bars and ride it that way for several days or maybe a week. Then when I know with certainty where I want my brake levers on the handlebars, I apply the handlebar tape last.

A side note - if you might want a handlebar bag, having enough steerer tube so that you can use a second stem for the handlebar bag is something best planned for in advance before you cut the tube. See photo for the second stem. That lowers my handlebar bag and gives me a little more real estate for other stuff on my handlebars.



I have since added some Aluminum strapping to keep the handlebar bag from sagging that badly.

If you can borrow an adjustable stem, that helps getting the fit dialed in to exactly where you want the handlebars for height and reach, then you can buy the exact stem you will want after you have figured out exactly what you need.