The rubber seal on the quarter glass slides out the bottom of the chrome piece. It is a T channel. Get the replacement set first and you will see how it interlocks. Get the set with the metal insert inside, like the originals. The all rubber seals don't work as well. Once you have the new seal, you won't be afraid to destroy the old one getting it out. I use a visegrips and start pulling down from the bottom. They seem to have some sort of glue or are rusted from age and can be very hard to pull out. By starting at the bottom and stretching the rubber, it will break free and slide out. Clean the metal track of any debris or rust. Lots of soapy water to keep the new one lubricated will help. It may take some patience to pull the new one up by hand and a small screw driver to feed it into the channel along the way.
Adjusting the glass isn't something I'm going to attempt to describe. It can be very frustrating with all the different adjustment that are possible between the 2 pieces. The quarter glass has the most adjustability to get it vertical and to stop at the right place for the door glass to seal. If you have a good local body shop with an old school body man, I would ask them how much to do it. You may be able to save a little on the cost by doing the R&R of the interior panels yourself.
1967 GTO; Gulf Turquoise on Black, 4 speed bench
1967 Camaro ss/rs 350
1966 Chevelle SS 396
1970 Chevelle SS 396
1952 Chev 2 ton flat bed
1938 Chev coupe