This part is fairly easy, but critical.

Place the fin in position, and line up the rear edge with the top rear corner of the fuse. mark where the fin needs to be cut. Just a little mark at the bottom will do.

Now using a square, mark the fin there and cut it.

using a sanding block and a bit more of that pateince, sand the front edge to fit snugly against the turtle deck face.

Once you have acheieved a reasonable dry fit, you can glue the fin.

Take care in alignment here. install the stabs, and use a square on your center lines. Make a center line mark up the turtle deck face so you can be sure it's sitting straight.

Mix up some 30 minute epoxy and glue on the fin. Spread it thin and even. It should make good contact, but not ooze out everywhere. Using masking tape, tape across the top of the fin and secure it to the stab tips. Play with you triangulation and squareness, you have plenty of time. When it's sitting good and straight, make sure you wipe away all of the excess eopoxy where the fin meets the fuse and turtle deck.

Allow this to cure thoroughly before removing the tape.

truthfully, I use 5 minute epoxy and a square. I get it close and as the epoxy starts to set, I tweak it into position until the epoxy kicks. It always comes out straight, but I don't suggest this until you've done quite a few.