If you talk to Isabelle, that doesn't do much. All it does is reset their shirt to a plain one.
The method I like best is to empty your inventory and keep only the shirt you want to give them, along with items they won't ask for like tools or flowers (if your inventory is full of flowers, they won't try to sell or give you random items). Talk to the villager whose clothing you want to change a few times and they might ask if they can trade or buy the shirt from you. If they don't, go do something else and talk to them again later. If you want to make it happen more quickly, you can use the diving trick (like you would for PWPs). In that trick, you just stay in the water for five minutes or more before going back to your town. If that villager is the first one you see, they'll ping you about something. Sometimes it's random questions or PWPs, but there's also a strong chance they'll ping you to ask you for that shirt. Doing it also lets you talk to them a bit more without them getting sick of you.
If you want to send the shirts through the mail, you don't have to send a bunch at a time, unless you don't mind the possibility of some of her furniture being replaced by shirts. You can do the mail reset trick: just set the time to something before a mail round (they're at 9 AM and 5 PM), save, and mail her the shirt. A few minutes after the hour, find her, and if you're unhappy with the result, reset and try again. Wrapping gifts in wrapping paper also makes them more likely to display them in their houses or wear it, so you should do that. I've noticed that if you send them a good letter (all the lines filled with something, making grammatical sense, a couple of symbols like stars) they're more likely to be happy with what you've sent them.
I hope that helps. If you don't really mind what she's changing to, you can just take the opportunity when another villager asks you to deliver something to her. Save and continue, deliver her the gift, and if it's clothing but she's not wearing it, quit without saving.
- - - Post Merge - - -
By the way, villagers are more likely to wear designs if you made them (instead of the Anytown designs) but it's more or less random which villagers wear it. I've noticed that villagers I have a higher friendship with are more likely to wear them but it could just be a coincidence.
I will try all of these tips!!! I'm familiar with the diving trick for PWPs, it makes sense that it would work for this too (good thing I just got a wetsuit on the island!). 
