Thank you! I would recommend starting with something like a card wallet for your first leather project. Believe it or not, a watch strap requires a lot more experience due to the way that the materials come together. Since watch straps are smaller accessories, they require more precision during both the design and making process. Either way, just starting a project and trying it out is the best way to learn! Here’s a post that I wrote about making an apple watch strap.
In terms of where I purchased the fittings for the apple watch, here are two of many places you can get them. The first is on Amazon where you can get a pair of fittings for about $13.00 to start like this pair. There are lower cost ones on Amazon as well which may be lower quality. I ended up purchasing mine from a manufacturer in China where they manufacture fittings for Apple watches. However, I had to purchase it in bulk. You can do the same using a website like Alibaba. Just make sure you do your research before purchasing!
I recommend starting with a pair from Amazon to see how you like the process before jumping to a larger order from a manufacturer.
Hope this helps!
Tim