I downloaded 3 fonts to cricut. When I use any of them to draw with they don't stay as a single line. There is a double line and hollow inside.

There are a few things you can do to overcome the "hollow" look when trying to write a thin font with Cricut.

1. Reduce the size of your text
2. Use a thicker tip pen or marker. This will fill in the space in some cases.
3. Duplicate your text in Design Space and have 2 layers of text overlaid on each other. Cricut will write over this text twice and do a better job of filling it in.
4. Use the Offset tool to create an "inset" -- enter a negative value like -0.02″ or -0.015″ and that will create a skinny shape inside your text that the pen will draw on a separate pass. Make sure to attach all of the layers before you make.

If you are looking for a curated list of thin fonts that write really well with Cricut machines and 0.4mm pens, check out my list: https://sniptoit.com/writing-fonts-for-cricut/