Once the stab is installed you need to rebuild the top of the fuse previously cut out to install the stab.
I use left over 3/8" sheet and a contour tool. Some careful shaping and sanding and you can get a nice tight fit.
After that, you add some 1/8" sheet to the top of the fuse and then install the fin.
I use 2 different references to get the fin square. (1) I draw a center line down the fin and I tape a weighted string to the top of the fuse.
make sure the fuse and horizontal stab is 0 degrees side to side and the string will hang right down the line on the fin when it is square.
I also measure from the tip of each stab to the top center of the fin. Make sure they are equal and the string is still right down the line and that's about as square and true as I know how to make it.

Steve