How do I make a skirt for my pencil Christmas tree?

I like to use an extra throw blanket and twist it around the bottom of the tree. It is easy and inexpensive so I already have the blanket

Here's quite a few ideas:

https://www.hometalk.com/search/posts?filter=Christmas%20tree%20skirt

I've used painter's drop cloth as a tree skirt. I just wrap it around the tree. It's all neutral color for all decor types and has great texture.

You can use a round table cloth and add lace. You can also decorate with your personal preference of embellishments.

I like to use tissue paper around mine, it easily adjusts to the shape i need.

https://www.youtube.com/watch?v=MccBS4BZd4w

You can do it the easy way and just wrap any type of fabric around the bottom. Or Cut out a circle in fabric using a pinking sheers