Shaping and thinning Crapemyrtle tree varieties, like the Southern Living® Plant Collection's Delta Jazz® Crapemyrtle:
Remove sprouts from the base and trunk of the tree.
Remove side branches that grow below 1/3 of the height of the tree. For example, if the tree is 6 feet tall, remove side branches growing from the ground up to 2 feet or if the tree is 15 feet tall, remove side branches growing from the ground up to 5 feet of the tree.
Remove dead branches and branches growing to the inside of the tree, leaving only vertical branches and ones growing outwards from the tree.
Cut back the stems that hold the blooms to 6" above the intersection where the stem meets a branch.
In the left picture below, the branches to remove and the stems to cut are shown in red. The right picture below shows the tree after the pruning cuts.