The best piece of advice I can give for making one-panel gags is to keep your ideas simple. Trying to condense a really long joke into one panel is just confusing and disorienting. Base your ideas off whatever you're comfortable with - real life events, conversations, interactions, random thoughts, etc. Keep a pen nearby to write down ideas as they come to you, and make sure you offer enough writing down so you can -remember- what it was supposed to be about. (I suggest pen because at least if you don't have any paper you can write on your arm or something lol). Honestly, I have a Notepad file with most of my comic ideas, but sometimes I only write like, one word per idea so there are a few comics already I've completely forgotten the premise of because I didn't elaborate a bit more than "Dalek" or "pizza" xDDDD
But yeah, that's the best piece of advice I can give - keep it short, simple, and to the point. If you must have a long joke, either make it more than one panel or make each panel on its own and each episode in parts (I've seen lots of one panel comics do the latter just to stay consistent with their style and make it less of a workload for them).
As for suggestions, I can't give you any that I wouldn't end up using haha so all I'm gonna do is leave you with that advice and wish you luck! <33
