Put the main triangle skeleton in a bench vice. Adjust the seat tube to vertical with a level. It will be close to 73 degrees but you are putting it at 90 degrees.
Establish the size of you frame. If you are building a 55 cm frame, measure 55 cm from the middle of the seat post to the head tube. Be sure you are on an exact 90 degree angle from the seat post. This will mark the middle of the head tube. Now is the time to tweak your build a little bigger or a little smaller if you need more or less reach.
The head tube angle will also be close to 73 degrees, but depending on the frame you are building will be maybe 1/2 to 1 degree plus or minus the seat tube angle. This is easy to find without angle gauges. If the angle is the same, just use another level vertically to mark the head tube angle. For plus or minus, refer to the diagram below.
Add about 1" or 25mm outboard of the center head tube line. Trim to this line. Dado the center of the poplar plug to the center line. You will use the bottom of that dado as a guide for a 1/4" drill to mark the dead center of the head tube sleeve. Drill up from the bottom of the plug, and down from the top with a long 1/4" drill bit. They should meet in the exact middle, but going at it from both sides can help goal seek on the center if one is a bit off.