Answer:
There are a few options.
1. un-hook your garters and pee normally. This is a pain.
2. Don't wear underwear. This is popular with porn stars.
3. Slide your panties to the side like you're wearing a one-piece bathing suit. For Number 2s, try pulling them down as far as you can without unhooking the garters or tearing your stockings and adjust them when you're finished.
4. Wear underwear OVER the garter belt. This way you don't have to take the garters off at all and your panties should help keep the them in place so you make fewer adjustments throughout the day.
Garter belts these days are less practical and more like novelty lingerie for sexytimes. In the past, they were made of sturdy cotton and had metal clips. Nowadays they are made of itchy lace and feature cheap plastic clips. This is a shame. Pantyhose tend to make some women look like sausages and require slips so that your dress doesn't stick to them. Stockings and Garters are much more customizable in regards to sizing and look good on nearly everyone. The downside is that you dress needs to be long enough to cover the tops of your stockings when you sit down. Unless they're just a novelty, try sitting in a knee-length skirt and pretend you don't know they're showing. If you do this in a bar, you will drink for free all night.