Armrest is a decorative block used in homes. You can use it to give rest to your arms.
This guide will tell you the method to use item frames as armrest.
At the start, place an iron bar on the ground and then spruce trap door at the top of this iron stand.
After this, add one more spruce trap door at the top in the same way as shown in the image below.
Add an item frame on both the sides of the spruce trap door. It will look like a chair now.