1. How often? Depends on how short they are now, when the last time it went thru a molt and how fast your bird grows it's feathers. Best way to tell is observe when it jumps off the cage or stand. If it glides down, then flying is not far off. If it drops quickly, then you have some time yet before a clips is needed. Bird I girl is suggesting you let the feathers grow until the bird can fly. You must make the decision.
2. Best is to have an Avian vet, not a general veterinarian , to clip the wings. So the list, in decending order of preference should be
Avian vet
General vet
Bird store technician
Pet shop technician
You
big box type pet supply worker
Me, personally, I would only consider my avian vet