You need to make a jig in which to assemble the neck, sides, and end block. It can be as simple as a flat panel with a block at the end and the outline of the uke drawn on it. The block on the end is to give you something to clamp the uke's end block against the sides. Note that there are also vertical pieces to hold the sides in place:
First, dry fit the sides into the neck to check the fit. Also check that you have a nice clean joint where the two sides come together at the bottom end. Make the end block - width of the sides, maybe 2" long x 1/2" thick. Dry fit everything in the jig to make sure it all fits. Figure out which clamp you are going to use to secure the end block.
The joint(s) between the neck and side(s) are secured with small wedges:
What you're after is to snug the sides tight against the outside edges of the neck groove so there is no gap.
Out of the jig but on a nice flat surface, glue the sides w/wedges into the neck. Then take that assembly and squeeze it into the jig. Make sure you've got plastic wrap or wax paper where glue might get on the jig. Make sure the sides are tight together at the bottom and glue the end block over the joint.
Next: Make & install linings
If you find any of this helpful I'd certainly appreciate a "tip" :-)