There is lots of advice on the internet if you type in something like "what to wear when shoulder is in a sling" or variations on that.
One site I found says:
"I've learned a few things from my recent shoulder surgery. Planning helps! What purchases and preparations can you make ahead of time in order to be ready?
"Next: a few things to consider ahead of time regarding clothing. I prepared several inexpensive large white T-shirts, slitting the seam of the T-shirt from under the sleeve down the underside (of the injured arm side.) This let me slip the T-shirt sleeve over my good arm, put my neck through the neck hole, and loosely drape the other sleeve over the shoulder dressing the first few days at home/bedtime.
"Later, a sweater put on your healthy arm and loosely draped over the injured shoulder is one choice. Once you’re past the painkiller phase, another choice is a garment with large armholes and neck hole, and loose, short sleeves. Remember, clothes go on the injured arm first, using your healthy arm to adjust the top, the injured arm supported at all times. Once the top is in place, put your sling on over the top. I went through my closet then went shopping the week before surgery. Trying on clothes with one arm in a pretend sling position before surgery is a lot better than trying to get into clothing after surgery and finding that it's too painful, or you can't get back out!
"The sling padding gets really itchy. I tried several liners but found a light silk scarf worked best.
"Before your surgery, purchase a package of Velcro with adhesive backing. You won't believe how many things you can stick to your bulky sling! (I attached my cell phone and sunglasses)"
I don't know if any of this is helpful but, if not, I'm sure you will find some tips that are useful if you do an internet search.
Good luck.