I always loved the idea of keeping a sketchbook but always found it really hard to do, it's a task that I really have to force myself to do. I feel like with other people it comes more naturally. I can never find the exact right kind of sketchbook either, there's always something I don't like about one I buy and I'll use the first 5 pages and then never open it again (I'm probably just making excuses).
Starting on the first page is always a very scary and daunting task. What if I mess up? What if it's not a cohesive sketchbook? I've learned to just relax and let it happen. It's a way to get stuff in your head down on paper, you're not starting a masterpiece (or maybe you are). Mostly when I draw I use loose computer paper because there's no pressure. I now have a stack of paper literally 4" tall of drawings like this. Seems like it makes more sense to just draw in a sketchbook, am I right? However you can wrap your noodle around it, you just gotta find a way to do your doodle.
Stuff I draw is usually a mix of nonsensical figurative line drawings, bag ideas, color swatches, little illustrated puns, etc. It's a combination of drawings neat and imperfect. Some are loose ideas that are refined later or forgotten and some are finished ideas that make it into finished products like a textile design on a hat, a sticker, an embroidery, or a finished bag or plush.