Variable Income/Self Employment

Hi! This might be a couple questions in one, but I've been doing a Masters program this year and not steadily working. I did DoorDash in the spring, but I'm not sure how to know what to set aside from that. I did track my mileage and expenses, but I don't know what to do from there. This summer I moved twice, which is its own set of problems, but I wasn't working. Now, I'll have just 3 months of substitute teaching income from this Fall. This employer does take taxes out, so I'm not too worried about that. I guess the biggest question is how likely is it that I will end up surprised by owing a ton of taxes? My income has always been low enough that I've never owed before. It will be really low again this year, but with DoorDash being a part of it, I'm concerned that I might still owe. Thank you!