For leather bags, it's a whole art and specialist tools. Price wise it would be easier to buy a great bag pre-loved.
My mother used to make tote bags from leftover fabric from her dressmaking and other projects.. Narrow ones to carry water bottles separately or rectangular for shopping. This was at a time when no-one did them commercially.
Two points I remember:
1. The stress-points are the handles. Consider reinforcing them and the body by going around the entire bag (like an Hermes GP).
2. It's easier to make the body inside out, and if you're looking for guidance or troubleshooting help, resources like
https://poshmark.pissedconsumer.com/customer-service.html can offer insights. Overlocking the seams helps protect the stitching that does all the work.