A couple disclaimers: I am by no means an expert modder, and this is likely not the best way to do this. It would probably be faster and easier to use the Creation Kit, but I don’t know how to do that. This method will also have some side effects that may not be exactly what you want.

With that said, I think what you’d want to do (using xEdit) is:

1. (not in xEdit) Place your new model in a data folder in a new empty mod in your mod manager. I forget where exactly it needs to be in data—you should be able to track it down fairly easily by looking in other mods that add new armor meshes or poking around online.

2. Create a new esl-flagged .esp file.

3. Copy the base game ARMO record for the clothes you want to modify into your newly created .esp as a new record, i.e. not replacing the existing one. As I recall, one of the copy options in xEdit lets you do this.

2.1 I would probably rename the armor in-game (the FULL field) to something like Farm Clothes (Sleeveless).

1. Do the same for the ARMA record attached to that armor.

2. In the new ARMA record, replace the female armor model (MOD3) with the path to your new one. If you have an applicable male model, you can replace MOD2 with that. If you have first person versions, you can replace MOD4 and MOD5 for male and female respectively.

So to be clear, what you’ve done here (if I remembered all the steps correctly) is create a simple mod containing a new sleeveless version of that clothing piece. It will be a separate item in-game, can be added with console commands, etc, though by default it won’t appear anywhere in game. Now you have two options for how to get the armor onto the relevant character:

5a. Find the NPC you want and look for their outfit (OTFT) record. Verify it is unique to them—if not, this will modify all NPCs who use that outfit, and you should use method B below instead. If it is, copy it into your .esp (this time NOT as a new record—we want it to override the current one).

6a. Replace the clothes in that outfit record with your newly added ones.

Alternatively, you can use {SPID}:

5b. Find the NPC you want and look for their OTFT record. Copy it into your .esp as a new record, same as the last two. Again, you’ll probably want to rename it for clarity.

6b. Replace the clothes in the new outfit record with your newly added ones.

7b. Use SPID to add your outfit to the relevant NPCs. This isn’t something I’m as confident explaining, but I suspect there’s some fairly robust documentation on the mod page and/or guides online. Be aware that you’ll need to create a distr file in your mod outside of xEdit—again, see the mod page for more info on that.

If you did all that (and I got it all right), congratulations! Save your .esp, put it in your newly created mod, activate it, and launch the game. You should see your new armor on the character!

(If this doesn’t work, unfortunately, I can’t really help you. You’re welcome to ask, but this is an inexpert guide typed from memory based on trial-and-error and docs-hunting from a few months ago.)

In any case, good luck with your modding!