How do you degloss old paint? Is it very difficult to do?

try paint thinner or sand paper

For interior paint, a light sanding works. You're painting over it, so you just enough roughness for the new paint to adhere well, not remove the old paint.

Autobody shops have degreasers and other solvents, but I suspect you're not painting your car.

Don't use acetone. It's flammable and the vapor can knock you out. TSP is lots safer & cheaper as well. As is sanding.

Acetone is the appropriate deglosser. Use gloves.

the professional method is to wipe the paint the with "Wil-bond". It made for this purpose. Follow label directions carefully and use with adequate ventilation.

Pre-cleaning with TSP seems to de-gloss many paints. Worked well for me on many projects.

Just go over it with sand paper to remove the high shine, scuff it up.