Step 1: CAD system Layout
You can also see the Center of Gravity (approximately the bellybutton) is about 1/3 of the wheelbase distance ahead of the rear wheel contact point. This good weight distribution leads to nice handling.
Step 2: Layout donor bike on CAD setup
Step 3: Layout the Boom once cut from the Donor Bike
The boom is positioned on head tube to see where to drill miter hole. The drill with attached hole saw** bit is positioned to convey the idea of mitering.
* Headtube is the pink frame's front frame tube through which the bike's fork steering tube passes.
** Hole saw bit is the 1.5 inch diameter white toothed cylindrical saw in the drill chuck.
Step 4: Drill the hole that makes the Mitre
Step 5: Cut and File the Miter for the Joint
Step 6: Use CAD to check chainlines, etc
Layout the boom and frame on CAD to look at Chain-lines (shown here with Yellow battens) Also look for leg extension and possible heel to wheel interference. e.g. sometimes my shoes get scuffed by the front tire while riding some bikes I make, but not this one.
Keep adjusting (cutting or filing) the miter to keep it snug against the head-tube when the boom is positioned where you want it.