When making applique blocks I like to cut my background fabric into a rectangle that measures a minimum of 6" x 9". For this block I also cut out my print fabric the same size. Then I traced the pattern onto the front of my background fabric using a water soluable marking pen.
Using contrasting thread - I hand basted the fabrics down about 1/4" away from the outside of my marked lines. Then I cut away the excess background fabric from inside my drawn lines (where my print fabric applique's should be) I left 1/4" seam allowances and clipped all of my inside points and corners.
Using a toothpick, I turned the first seam allowance under. If you are new to needle turn applique' - you might find that toothpicks work easier for turning, rather than using your needle.
The toothpick also helps you iron flat your seams when you rub it against your turned edge.
To get nice clean corners, I like to take an extra stitch "head on" into the inside corners. Once you've added that anchor stitch, turn the next side under and continue working your way around the diamond.
Once you've finished the reverse applique' on this triangle - the block will look like this:
Center your triangle ruler over the block and trim