Hi, yeah, I've done that often, you should use core soap or soft soap, shower gel or dish soap are not so suitable, they have other additional ingredients. Then take about 20-30 ml of liquid soft soap or 20 g of solid soft soap in one liter of lukewarm water. Optionally, add a little vinegar, but only very, very little. I would only do that if you really have a lot.
If they are standing directly in the sun, then in the evening. But if you can do it early in the morning and there’s no sun shining directly on them for the next few hours, that works too.
Or you can look for some ladybug larvae, they love to eat aphids, if you find some in the garden, they will go straight for the ladies. Good luck and kill those pesky bugs.