I’ve been making leather watch straps for 10 years. I am sure that my experience will be useful to beginners. If you want to make watch straps, the first thing you need to do is buy buckles.
Watch buckles come in widths from 10mm to 28mm. (in multiples of two: 12, 14, 16 and so on). For a beginner, it makes no sense to buy all sizes of buckles at once, just the most popular ones.
For your convenience, I have listed the most popular sizes (widths) of watch bands in the comments. But you can But you can argue with me!)
I have a dozen watches in my collection. Half of them have straps with a width of 20 mm.
The majority of watches have strap or bracelet widths of 20 or 22 mm. Vintage and military watches are more likely to have narrower straps: 16 or 18 mm. Panerai watches have a width of 24 mm. Not all Omega and Longines watches have non-standard strap widths: 19 and 21 mm.