I've made enough batches to know that most people mess this up by mixing everything and then tasting - you end up chasing your tail trying to fix it. The thing nobody mentions is that you should taste your lemon juice straight first. That tells you what you're actually working with. Some lemons are pretty mild, others will pucker you up immediately. Once you know what you're dealing with, you've got a baseline.
Start by juicing your lemons and diluting with water at something like 1 part juice to 4 or 5 parts water, then taste it plain. From there add sugar gradually - and I mean gradually. A tablespoon at a time, stirring it in and tasting. You'll find the sweet spot where the tartness isn't aggressive but you can still taste the lemon. Bottled juice does work in a pinch but it's flatter and sometimes has that chemical edge, so fresh is worth the effort.
The other thing that helps: chill it before you decide it's perfect. Cold lemonade tastes less sour than room temperature because cold kind of masks acidity. If you tweak the sugar while it's warm, then chill it, you might find it tastes off. So make your batch, let it sit in the fridge for a bit, then do a final taste test and adjust if needed. That way you're not over-sugaring it trying to balance something that just needed to get cold.