arfer is a bit unusual in that it is a noun and also a verb-noun:
- As a noun - a custom, habit, practice, usage
- As a verb-noun - to become used to, to be used to
If you look it up in www.gweiadur.com or a similar dictionary you will find examples of it in use.
Its use with the imperfect to mean 'I used to xxx' is not really necessary, as the Welsh imperfect includes that meaning anyway, but it is a common pattern in the colloquial language:
- Ro'n i'n arfer gyrru i'r gwaith - I used to drive to work
- Ro'n i'n gyrru i'r gwaith - I used to drive to work, I was driving to work
'to make to order', 'to custom-make', 'to make to meaure' might be gwneud ar archeb, gwneud ar fesur or similar.