I've posted my home made washi tapes a number of times before.
I use the "plain white paper tapes" from the first aid section of WalMart or any Dollar store or Walgreens, etc. I use inexpensive craft acrylics for this and just have lots of fun using stencils and stamps and fine line paint etc. I love the brightest colors possible.
In this example I laid all the tapes out on a piece of left over foam core. The paint soaks though a little bit. You can do this on wax paper, freezer paper (my favorite) or a piece of plexiglass or even an old plastic table cloth or place mat. Just so it will come up easily.
Here I show that I am beginning to roll up the dry tapes on to a drinking straw...I use the larger "milkshake-type" straws. It's a good way to keep them for later use.
Here is how the straws look when you are done. Then I just plunk them into an old mug or can and voila they are ready when I need them for collage, stationary, envelopes, or any fun use. I like these SO much better than the commercial washi tapes.