How to Finish a Song in One Evening: A Realistic Plan

A plan for about three to four hours

Beat (20 min). Choose it. Find the key and tempo with the free key finder and BPM finder.
Record (60 min). A few full takes, then fix the lines that need it.
Clean and tune (30 min). Remove noise, fix the worst notes, and de-ess.
Balance (30 min). Set the vocal against the beat and add a little space.
Check loudness (15 min). Use the LUFS meter and listen on a phone.
Export and stop (10 min). Save the file. Listen the next morning before changing anything.

Time depends on the song and your experience. Stopping on time matters more than any one setting.

Spot clipping, noise, rumble and level problems. Runs in your browser, nothing is uploaded.

What if I run out of time?

Export what you have. A rough, finished version beats a perfect unfinished one.

Should I mix the beat too?

If the beat is finished, leave it. Focus on the vocal and the balance.

What is the first thing to fix?

Noise and the worst notes. Small details come last.