At this point, the ukulele neck / body assembly is somewhat flexible, and the neck may even bend slightly down. Gluing on the top stops all that and makes it rigid, so it's important to proceed carefully.
Dry fit everything in whatever clamping apparatus you've devised, and place a straight edge along the entire length. Shim the neck up so that it and the body are in the exact same plane under clamping pressure. Tape the shim on the work bench or jig, and then take things apart in preparation for gluing. Apply the glue & clamp.
Next: Trim top & bottom edges
If you find any of this helpful I'd certainly appreciate a "tip" :-)