Machiel

Hi guys, I'm a newby to leather hand working. I've made a a few wallets thus far but seem too struggle to keep an edge straight. I cat cut a straight lines out of the leather with a rotary blade but the moment I stitch the edges it is as if it pulls the leather in and therefor makes for bended lines? Can you please assist me with tips on how to solve this and create a neater product? Looking forward to hear your responses. Kind regards.