I've never done a barn door, but I've done the windshield and rear glass. It takes a bit of patience. First of all, you need glass that actually fits. A lot of times it is a bit oversize from repeated tracings of original glass, and it won't fit no matter what you do. Hopefully yours isn't.
Second, you need rubber that actually fits. Steele is the very best. I used Chevy Duty's rubber this time and it went ok. You need to do it on a warm day so the rubber is nice and pliable. Leave it in the sun for a few hours.
When you have the glass out clean the pinch weld really well and apply a coat of your favorite caranuba wax. This will both protect the paint and make the rubber slide easily.
Put the rubber on the glass first, and when you put it in the door, start with the tightest radius curves (bottom on the windshield) first. They are really tough to get around when the glass is in there tight. Wrap a piece of parachute cord all the way around in the rubber. Overlap it a little, even. Have your helper hold the glass in place from the outside. I was able to do the windshield by myself but I have long arms and a good vocabulary of swear words.
Use a putty knife to assist you for when the string isn't getting it, and use it to relax the rubber once it gets over the pinch weld. Take your time working the rubber over the flange, it tears easily if you pull too hard with the rope. Slowly zip the cord around, pulling the seal over the pinch weld and using the putty knife to coax it into place. If things are going badly, pull it out and start over.
I save the sealant to the last step, using the very narrow tip of the caulk gun to force it under the rubber until it oozes out. I have not had any luck applying it before putting in the glass.
