Thunderhead. I agree with using two channels and then mixing. I have a Futaba Super 8 Tx. so I can't help you with a JR. That said, what I would suggest is to find one of the guys at your field that does know how to mix channels on a JR Tx and then get him to help you. This is one set up you don't want to get wrong for your maiden flight. Make sure this is set up right and remember to not get too agressive with the throws. It will bite you and snap if you set too high and then yank back on the elevator. I hesitate to add this because again this is how I have mine set up for a Futaba Super 8 Tx. You should check these settings against the directions and your JR TX.

First make sure each servo horn is centered to the servo with theTx. trim set to neutral. You probably already know this. Just making sure. Then set each elevator to a neutral level setting to the horizontal stabalizer. Make sure they are both set mechanically the same. I then mixed channel 2 and 7 with 80% throw setting at the radio. Check the directions for up and down throw range mechanically to the directions, and then see if this Tx. setting accomplishes the desired throw up and down. Each elevator should move at the same speed with the same up and down throw movement. Exactly the same.