I need to make a disclaimer here before I go any further on the subject of asexual reproduction: Like any clone, the clones that you create, will have the genes of the parent, and will consequently have all the good and bad characteristics of the parent plant.
If you have a disease problem with one, it will also attack the others! The more "specific" any line becomes, the more susceptible it becomes.
The Irish potato famine and the North American Chestnut blight are good examples.
It is OK to reproduce your plants this way, but always understand the need to keep some diversity in your garden, and don't put all your genes in one basket.
"Variatiodelectat"- "Variety is the spice of life".
Now, A "hardwood" cutting works well with crepe myrtles, (softwood cuttings of many species can be rooted in a glass of water) hardwood is wood that has become, well, hard, or "herbaceous".
There are many theories on when the best time to take a cutting is, but with the crepes, and good watering conditions, I have done this year round.
Make sure that your soil is prepared beforehand, and you can use a "dibble stick", to poke a hole for planting.
Good, sharp pruners are a must, and freshly cut branches are a must.
Take a cutting, about 4 to 6 inches long, with a bud node about a quarter of an inch from the bottom, and a couple of buds on the higher part of the cutting.
Do this at an angle.
Check your cutting, ragged edges almost insure a less than satisfactory outcome.
I keep a sharp knife handy, to trim any ragged edges away.
If you are using rooting hormone, dip the big end into it.
Then place it into your planting hole in your soil.
Gently pack and repeat, until you get the numbers of cuttings you need.
Always do a few extras, because you will lose some of them.
Water well, and make sure they never dry out completely.
As time goes by, you will see shoots developing, and hopefully the roots are doing the same.
Callous will develop on the bottom first, followed by adventitious roots.
Leave them in the planting medium for the first year, and then transplant them into pots or the ground.
As you watch them grow, pat yourself on the back.
You have done something that is a tradition as old as mankind, and as new as genetic engineering.
Enjoy!