If you have the proper tools you can DIY...not too difficult actually.
I regretted sending one of my watches before for resizing and it was scratched at the clasp by the AD...
Unless you buy it new from the AD - they resize it when it is still fully stickered reducing the risk of scratches. If you bring it in after you've bought it when the stickers are out - the risk is higher.
That being said, if you are unfamiliar, then going to the AD is your best bet, or any careful watch maker/dealer.