The tongue should remain relaxed in your mouth. It is not doing anything at all to produce the single note!
I’m not sure how long you have been playing? but the key to getting the pucker right, is repetition and patience. It can take up to 3 weeks before it starts to click in, as your muscle memory needs to be retrained to remember the shape your lips take. When I originally started this course I would suggest not moving in between lessons for 2 weeks, as in my experience it can take this long to get things right at the beginning.
However…. having said that, In this case I would suggest moving onto step 4 as I introduce improvisation and the are a few acoustic tracks I have uploaded for you to play with. When playing with these tracks it’s ok not to be so precise, and they can be a lot of fun too.
You need to strike a balance between the fun and the ugly practice. The good news is the fun starts to flow much more after the initial single note and bending skills are down!
Hope that helps!