Here is a short description of how I setup my neck assembly jig to cut scarf joints for a typical 14 fret to the body, acoustic guitar with a 25.4” scale.
First place a mark about 350mm (13.75”) from the right inside of the side fence to the cut line on the front fence. This will be outside of the cut line of the scarf joint by the band saw blade. Transpose this mark on the top of the fence and on the outside face of the fence. I usually take a small try square and mark a vertical line of the inside of the fence then draw a line at 15.11 degrees on top of the 19mm fence wood and finally another vertical line on the front face of the fence. This final line will be the entry point of the blade to start the scarf joint.
Next mark a straight cut line from the bandsaw blade directly on the table of the bandsaw all the way to edge of in-feed side of the table. this will be your straight cut line that we will measure the scarf cut angle.
Place a angle try square or other device to measure angles and mark out the 15.11 degree angle cut line for the scarf joint. on the bandsaw table as shown in the photo below. Note this line will angle out from the blade to the left as you face the infer side of the table.
Now set the assembled jig on the bandsaw table as shown the the photo slide show below, and place the front face of the fence against the blade teeth and angle the jig until it lines up with the 15.11 degree line you drew on the table. You may want to clamp the jig in place at this time to hold it firmly in place. Make any adjustments as necessary at his time.
Now slide the maple miter gage insert you received with the jig into the miter slot at the outside of the table, or to the right as you face the in-feed side of the saw. Keep the maple insert sticking out just a bit and place a mark on both sides of the stick on the edge of the jig base.
Next slide the stick so it sticks out the opposite side of the jig and make both sides here also. Check the lines to make sure they are very accurate.
Unclamp the jig from the bandsaw and flip it over. Transfer the lines from the edges to the bottom of the jig base to the bottom surface of the base. You should now have 2 parallel lines that mark the outside edges of the miter slot insert on the bottom of the jig.
Clamp the pre-drilled maple miter slot insert on the jig bottom with a clamp at each end. Check that it lines up with the lines exactly and adjust as necessary.
Chuck up a 1/4” drill bit and carefully drill through the jig base, using the holes in the maple miter guide insert as a guide. Keep the drill vertical if you can. You may want to clamp a piece of plywood on the exit side of the hole in the base to prevent blow-out of the drill bit.
Now attach the maple miter slot insert to the jig base using the 1/4 machine screws, washers and 1/4” free-spinning knobs supplied with the jig.
Feel free to use wax or lubricant on the guide to prevent any sticking points. Also the guide is sized for the miter slot in my bandsaw and you may need to make some slight adjustments. If there is too much play, glue a piece of maple veneer on the edge of the insert and sand to fit accordingly. If the fit is too tight, carefully sand the edge of the slot with slow long strokes while the insert is clamped in a vice.
After the insert is attached to the jig base you are ready to make the first cut through the new fence. After that, you are good to start cutting the scarf joints on actual neck wood.