Such a small thing can improve your printing quality , or even transform a not useable assembly into a fully workin printer !
If you follow the cobblebot's how to , you will either have wobbly idlers or fully fixed ones because of the bearings sleeves end outer rings being pressed against the mouning plate.
You will need one big and two smalls shims for each idler, fit the big one in the big bearing and put the screw in it then a small bearing and a small shim and do it a second time. You shall have no bearing touching any other (or slightly, if it spins freely when tight , then it's a succes) .
You may have to do some filing/scaling , such small object can be hard to print with acuracy.
For owner of the cobblebot only : If your steppers keep skipping , try putting a lot of lube on the belt and , if it still doesent work, increasing the current limitation of the drivers (blow on them with a fan if possible) , this upgrade shoul remove your issue.