First of all,just like Dala said,buy the upgrades to your tool from the leatherworking trainers.It should make the job far easier.
Then,In order to get rare quality of anything(for leatherworking) when you're crafting,you'll need to fix any blue leather roll and just neglect any other colors so only lock blue leather rolls and then you should find a button that enables you to cut leather again if you have upgrade,click on it and if you get more blues lock them and cut clothes again if it allows you to do it.
It's about luck in getting full blue leather combination so you keep trying till you get all the rolls blue color at some point and when you do it,just confirm to craft it and you should get rare(blue) quality of the object you're trying to craft through leatherworking.
If you need a certain quality,you should try to get full leather roll match of this color only except for common quality.
The lowest quality is white as far as I remember.
Common quality is green.
Rare quality is blue.
Epic quality is pink or purple.
Legendary quality is orange.
Mythic quality is light blue or turquoise.
Though I doubt if you'll find any option for the last 2 qualities when crafting anything so you should just memorise the green (common quality),Blue(rare quality) and pink (epic quality) mostly since they're the most common colors you'd find while crafting in leatherworking.
I hope that helped.