That is the crux of the project, easy to screw up
Just sew the last flat felled seam through the previous flat felled seams. You end up sewing through many layers of fabric. With thin fabric not a big problem
Start at the bottom and sew up to the peak, then it’s easier to deal with that difficult part
Have a seam ripper in case you accidentally “capture” a layer of fabric you didn’t intend. After doing this occasionally, I have more muscle memory to avoid repeating.
I sew a circular reinforcement at the top. After you sew the regular seams, set the tent up. Not really taut or in the wind so it doesn’t rip. Then place your circular piece over the top and do a hand stitch through reinforcement and each flat felled seam. You have to make folds in the circular piece to get it to conform to the tent. That is, turn the circle into a cone. Stretch the circular piece a little.
Then take tent down and sew a couple rows of stitches around the circumference of the circular piece (now a cone).
This hides the peak where it might be a bit messy, both aesthetically and strength wise.
That is a way to do it