How can I use soft pastels with an easel?
You use a drawing board. When I was in high school we used these for our drawings and paintings all the time whether using an easel or sitting at a table. It allows you to easily move the paper around without touching it as much and gives you something to hold onto. I remember we taped the paper down to the board to keep it in place.

Here's an example of a similar easel on Amazon and it includes an image of a drawing board to illustrate how it's used (it doesn't include the board).