Start slow & be patient
Once we know the feeling of proficiency on one instrument, it's difficult to recall what it was like to be a beginner. Oftentimes, when you pick up a new instrument that you aren't proficient on, it can be frustrating to be unable to execute the ideas you hear in your head that you could easily execute on your primary instrument.
In this scenario, the instinctual response is to try and force the ideas out and push to play beyond your limits on the new instrument. This is a very dangerous approach; at best, you'll develop a lot of bad habits, and at worst, you could develop performance injuries that might restrict your ability to develop on your new instrument, and possibly even damage your abilities on your primary instrument.
Take it slow. You're a beginner again; relish it! Play your scales at painfully slow tempos and shed on "Mary Had a Little Lamb" until your ears bleed. It'll be worth it.
(Resource from the web)