Hoi An is known for its tailored made clothing. You'll find NUMEROUS shops when strolling around the old town and it does get overwhelming if you want to find a good one. So I would def recommend doing your research beforehand. I went to "Milan" and this place is known for its linnen. Each shop specializes in different materials.
Once you know which shop you want to go to, you can do some research on what exactly you want to make. I had some pinterest posts saved and just walked in, showed them what I wanted and they immediately start measuring you! The process takes around 30min depending on how much you want.
Do ask them beforehand how much it's going to cost you! (You can pay by card but you'll probably have to pay a bit more).
TIP: Go to the tailor shop the first day you arrive in Hoi An. Usually your first fitting will already be the day after, but if you have to get a 2nd or 3th fitting it could take up to 2-3 days untill everything is ready.