As seen in photo 1 you will need to glue the sides down to one side of the book. (Hint: I ironed the sides in half. Make sure you have the sides halved with the sewn middle line on the inside. The whole purpose behind cutting the template in half for the sides and then sewing the two half triangles together was to make a crease that would allow for a cleaner close.) I've added close ups of these steps below.
Next add glue to the entire inside panel you just glued your sides on. I dabbed my glue all over paying special attention to the edges.
Then add the first half of your inside panel. Don't do the spine yet.
Let this dry for a few minutes before gluing the other side. (Hint: Place several books on top of the glued sides for a few minutes to make for a stronger hold.)
This next part can be a little tricky. If you have some paper clamps you may want to grab them now and use them unless you've a third hand...
Dab glue on the entire half of book you have remaining. You will want to put a line of glue down both sides where your side fabric will be and down your book spine as well.
Then very carefully place the side fabric triangles to the line of glue on the other side and tuck in the bottom of the triangle fabric on the spine. Add the inside panel and clamp with paper clamps.
(Pictures show detailed instructions. Please refer to them if you are confused or just ask and I'll try to walk you through.)