Well, I would say that first of all think about what you're prioritizing in the work. Do you want a piece that's mostly emotions/storytelling, or one that focuses and prominently displays specific visual details for the sake of historicity? Most pieces have a mix of both, but just like movies vs. history books/documentaries, there is usually a focus on one or the other. Think about the artist's strengths, what draws you to their work, and see if you can incorporate that into your request. If your focus is more on drama, and if you trust/collab with your artist, leaving a bit of wiggle room for artistic license can really make something cool! On the other hand, if you want to make something more clinical/specific, you'll want to be a bit more involved.

the main practical thing you can do for your artist is gather REFERENCE! if u compile it all yourself in a folder or something you will make their lives SO much easier. This is for me personally, but also if you are writing detailed descriptions, PLEASE add line breaks/bullet notes/paragraph breaks because it makes it much easier and faster to read and find information. For historical stuff, If you can't find like a good example of a particular garment, find something that's similar and include that as well (for example, even a model wearing a plain tunic can help with seeing how something might drape even if it's not Super Specific or bedazzled)

If your focus is on something Dramatic or emotional, finding movie stills or comic panels or poses, even drawing your own stick figure arrangement can help the artist visualize what you're going for and how to communicate the personality/relationship/etc.

Don't be afraid to ask for changes etc in the sketch stage--all artists have different TOS in terms of revisions, but most ppl taking commissions are glad to get corrections etc EARLY ON in the process, rather than later when it's more finalized and harder to change--remember, you ARE a customer and it's your job of course to communicate what YOU want, not leave it a guessing game on their end, which can cause frustration for everyone.

TLDR; BE respectful, communicative, and use lots of reference, not just for the visual details but for the feeling you want to get across from your piece. Also, if u like ur piece, feel free to leave a tip ;D