I've done it by putting the seal on the screen first. I dont use the string method. with 2 people put the windscreen and seal into the frame and ease the lower edge over the frame with a flat screwdriver. Once the bottom is in give it a push down, then start to inch up the sides with the screwdriver, it doesn't need much force to get the seal over and be carefully not to scratch the paint work.
dont use washing up liquid as it contains salt and will rot under the seal.
Also use the best quality seal, makes a big difference, VW status are the best with shaped corners, the JK ones are utter crap and made from hard rubber.