Here is the horizontal stabilizer skeleton in the jig.

The jig is made of wood, and it basically provides a stable structure to hold everything in alignment.

After building the jig, you use plumb lines, levels, measuring tape, etc to get all the spars and ribs properly aligned, then you clamp them in place.

Then, you drill the holes for the rivets that hold the skeleton together, deburr the holes, and rivet the works together.