I used to go to Hello Laundry, an expert tailor near me in London for any clothing alteration service. The clothing alterations service guides me and makes me aware of some things. So below are the few tips which are given by clothing alterations service.
Keep in mind that no one likes to work in dirty clothes. Before handing your garment to the Alterations, make sure it is clean.
If in doubt, go with the larger size. It can be reduced in size in most cases, but making it larger is much more difficult.
Be aware of the risks. Your clothing, like every craft repair, may have hidden surprises that the alteration will reveal. Colour changes, old damage, maker's marks, and other details may become visible.
Know what your expert is trying to tell you. Ensure you understand what is done, how much that will cost, how long it will take, and the expected output. It may look like that tailor is speaking another language at times. If you don't understand something, ask again till you do.