If you didn't install the datapacks properly yet, have a look here.

1) Get the Datapack Control Book

Each datapack has a function to start with, noted on the armor stand creations page.

Here's an example command:

/function #mch:mch000001_steve-and-alex

Run the function and you'll get a book with further instructions on how to summon the armor stand creation.

2) Summon the armor stand creation

Open the book and click on "[Summon Object]". As long as there's no other similar but unfinished armor stand creation nearby, you can summon it.

Right after you summoned the armor stand creation, you can modify it with the chat panel.

If you don't want to have all the command feedback spammed in your chat while modifying the armor stand creation, you can change the following gamerule to surpress this:

/gamerule sendCommandFeedback false

When you brought the creation in the final position, you should finalize it in order to not accidently modify it.

Therefor, open the book again and click "[Finalize Object]". There should be green particles around the object and you won't be able to modify it with the chat panel any longer.