This install is based on the fact you have your windshield already removed which would mean the header and post finisher trim pieces along with the cowl are also already removed.
You’ll need to remove the door seals and front section of the headliner which will require removal of the sun visors. You will probably have to cut through and remove some old silicone sealant that may run over the post trim under the inner door seal – mine at least had a ton of it. At this time it is best to begin taking photos of how the old trim is installed noting where it has been trimmed at the factory. Here is my original trim.

I used a “pick” tool in order to peel up the old trim from the area under the inner door seal. The dash does not need to come out if it does not need to. I would suggest installing the post trim while the dash is installed so you can get the trim even with the dash. I removed the dash AFTER the post trim was in so that I could refurbish/cover it and make it look new.

With the old trim out, clean up the fiberglass install area and put some tape over the wiring that runs up the passenger side post if the old tape has turned to dust from age.

Test fit the new trim pieces. I had to force them as low as possible into the dash in order to make them uniform and even with the dash height. There was a bit of excess fiberglass on my car’s A post that needed to be broken off and filed down, your car may vary but you may also need to adjust during this test fit in a similar way.

Brush on contact cement [or your favorite upholstery adhesive, I stay away from sprays since they are so messy] to the fiberglass and material in the areas where the material will contact the fiberglass post/body. I did this in sections – the full inner most part where the windshield would install along with the upper, outer most part where the seal will install were first. I then did the lower part where the trim makes odd wraps around the odd angles of the inner door seal.

Use a few layers of contact cement and allow it to dry per the instructions prior to mating the two surfaces (material to fiberglass). Pull the trim tight before mating and finally mate the sections and apply pressure so it will stick. I used sections of old door seal to hold the trim to the door seal lip.

When it comes to the lower section you will need to wrap the bottom up and again pull the trim tight so there are no wrinkles. Again use clips, old gasket, etc in order to hold them together.

Trim the excess material off. Clean the fiberglass of any excess cement in order to be prepared for windshield install.