(I also link a video to 50 mark making ideas at the end of this YouTube video--I thought it might give you some ideas on techniques you may want to try to use when you fill in your sections!)
1) Before working on the actual paper I'll be using, I like to draw the number of sections I'll need on scratch paper. This isn't to copy it, it's just so I have an idea of what 21 sections visually looks like, how large the spaces are, and how the space of the paper is used. This step is optional but I do find it very helpful!
I use the same process as on my actual paper. I start off with a pencil and randomly drawing straight or curvy lines or a mix of both and purposefully leaving some sections bigger. Once I feel like I have created a decent amount of sections I stop and count how many there are. As I count each section, I write down the number in the section. Let's say I count 18. That means I am 3 sections short. So I will split up the larger sections I have to create those additional 3 sections to get 31. I take a look at it and move on to my actual paper.
2) I repeat the same process on my actual paper. I use pencil and try to draw the lines as lightly as possible to make erasing them later easier.
If you are only using straight lines, use a ruler and make sure the lines are straight and even at this stage.
If using curvy lines, I like to free hand it and draw the lines randomly.
Once you have created 21 sections, take a look and see if you are happy with them. If you see some that are maybe too small or too big, you can erase the lines and adjust them. Once you are done with this step, erase the numbers you wrote down (and this is a second time you can make sure it's time to add the tape!
3) Use your pencil lines as a guideline for where you will put your tape. I try to put my tape directly on top of that line. If you accidentally miss in some spots, that's OK, you can erase any pencil lines that show after this step.
For straight lines, follow the pencil lines as close as possible to keep things straight and neat.
For curvy lines, gently guide the tape to fit the curves--there will be folds in the tape and it won't be smooth. See video for tips / examples.
4) After the tape is down, press it firmly one last time to make sure it is stuck on well. Check to see if there are any pencil lines, smudges, or numbers that are still there and erase everything. Now you're sectioned paper is ready for you to fill in! :)
I tape the tape off at the end because I like the white borders but if you like how your tape looks, you can leave it on!