How to use a thimble

When I saw this in my book "Successful Dressmaking", I thought "Oh, that's how it's done". So, you're probably thinking I've never used a thimble ........ I have and I own one. I do however remember not knowing how to correctly use it and mucking around until I finally came up with the idea that's illustrated (I think I did anyway - it was quite a few years ago). In the intervening years, I'd apparently forgotten again. Which leads me to another thing - I recently had to use my thimble and it's way too small. I know I would have bought a cheap one and I'm pretty sure that at the time I didn't have a choice of sizes. Maybe it's one of those situations where you need to pay more for a 'custom' fit?