Assemble the windshield off of the car—windshield, then rubber, then trim. Leave off the corner pieces.
When installing the rubber, stretch the rubber in the center of the window to relieve any tension on the corners. This is important. Do not pull the corners on last, do the top center last, stretching the rubber in the center, only enough to "roll" it into place.
If you have aftermarket rubber with glued corners, before putting the trim in place, cut any excess glue out of the trim slots. Make certain that the trim slot corner is completely clear, smooth, and unobstructed. Do the same for the body slot. If the glue job has the corner misaligned, you can try to help.
Another comment about the after market rubber: Cutting the rubber extrusion to make the corner is not a straightforward process. Looking at the extrusion from the side, the cut for the windshield is about 60 degrees. Looking down at the window slot, the cut is NOT 90 degrees. It needs to be less than 90, so that there is more rubber on the outside, or else the corners will tend to come unglued on the outside.
Put the cord in the body slot of the rubber, ends at the bottom, overlap 3 or 4 inches, then lubricate the cord and the rubber flap of the body slot. Mild dishwashing liquid will do, but there are other things that work. Thick cord is better than thin, for if the cord is like string, it may tend to cut the rubber. I have heard of 12 ga. Insulated wire used for this.
Lay the assembled glass against the opening, centered. Take your time and be sure it is centered.
Now you must have a way to keep it centered. It takes THREE people one on each corner to keep the assembly centered and pressed against the opening, and a third inside to pull the rope. If you want to do it with only one person, you need to hold the windshield in place. I use a three inch ratcheting tie down strap, wrapped around the glass and door posts. I hold the strap in place on the glass with some duct tape, so it won"t slide as I tighten it up.
Pull the rope across the bottom of the windshield, pulling the rubber flap to the inside of the car. Take your time. Pull evenly, so both sides install at the same time. As you reach the corners, you may want something like a cotter key tool to help draw the rubber flap inside. Before continuing to pull the rope up the door posts, get outside and use your hands to "slap" the windshield down and seat it in the opening. Don"t be timid, the glass is stronger than you think. This step is important.
The rest is easy. Pull the chord up the door posts. Go outside occasionally and "slap" the glass to seat it. Pull the chord across the top. Install the corner trim pieces, and wash off the excess soap.
I have probably done as much windshield and backglass work on Type 34s as anybody. Troubles come from not keeping the assembly centered when installing. Only two people with a new seal and no other help will have trouble. It takes three people, or some other method to keep the glass in place.
As you pull the cord, it is important to seat the assembly against the body, by pushing in and down. The VW shop manual recommends the "slapping" method, and it works.
Dry installation can be trouble. Lubrication helps to assure that the cord will not cut the rubber. It also helps the rubber fully seat on the body.
If you have the time, let the assembly sit in the sun for a while, even a week. It will help to mold the rubber to the glass. Some people think this helps a lot. It is probably better that the rubber be warmer than colder, but I have put them straight in on a cool day.