Set the position
Place the reinforced area where the needle will meet your hand, often across the palm or base of a finger depending on the design. The leather should sit flat without a folded edge pressing into skin.
Adjust the strap until it stays in place with a relaxed hand. If the fingers tingle or the palm feels restricted, loosen it before stitching.
Practise the push
Use a scrap of the project cloth and practise advancing the needle with the protected area. Start slowly so the needle meets the leather squarely rather than sliding toward an edge.
The thimble should reduce force, not encourage you to drive the needle harder. Keep the cloth supported and let the non-thimble hand control the layers.
Maintain the leather
Keep the thimble dry and store it away from sharp tools that could cut the surface. Let it air after a long session rather than sealing damp leather in a project pouch.
Check the stitching and strap before each use. Replace or repair it when the needle can reach your skin through a worn spot.