Before sewing can begin you need to glue the front piece and the gusset together. To be able to the it accurate lay the front piece on the table and cover it with wax paper. This way you can lay the gusset on top without having to worry about the glue bonding in places you don't want. Start at the top of the gusset working you way down just around the corner. Then begin with the other side of the gusset and do the same. Take care that the edges align. This way you don't have to sand too much to get a smooth edge.
The reason why you glue the bottom last is that if the gusset is not the exact same length as the side of the front piece you can hide the extra (or less) material by forcing the leather to either stretch or compress a bit.
When the cement has dried sand the edges, scribe a stitching line around 5 mm from the edge and mark the holes with the pricking iron.
Sewing begins the top of the gusset and the thread should be long enough to reach beneath the second strap holder. This way you can hide this change of thread under the strap and only use two threads to sew the entire length. The reason I don't sew using only 1 thread is that is becomes too long.
Marking the holes and sewing took 1½ hours just for reference.
When you have finished sewing you can bevel the edges both front and back and burnish the edges.
Scribe a line around 5 mm from the edge.