Really all you need are some thin strips of patterned paper or thin card, some thin double sided sticky tape or even the roller tape (personally I stay away from wet glues for paper, it wrinkles when it dries and it takes forever to dry).
Using my chosen paper, I cut a nice straight edge for the length I need. Apply the tape then cut the other side straight. Peel off the backing when needed. If using a roller tape (eg Tombo brand) of course apply when the strip of paper is needed.
It’s great if you have a roller cutter or guillotine but you can use a rule and craft knife or scissors.
If you wish you can stamp and colour in your strip of paper.
You can get the kids to colour or paint some paper. Use some of their artworks from day care or kindy.
When you buy washi tape it’s mostly for convenience. I switch between using both bought and home made.