Use a factor 50+ suncream on your arms AND cover your arms when tanning.
It'll take a few weeks of sunbeds to even out. Try to minimize natural UV exposure to your arms in that period.
As be careful with your face, neck and genitalia. They also darken a lot quicker - you don't want to miss them darkening beyond your desired colour while you worry about your arms.
As they start catching up follow the same deal as with your arms.