As with most all projects, in the correct weather, it is most enjoyable planning the project while snuggled up in pure wool blankets for sale.
As soon as the cabinets are installed, measure very carefully for the granite counter top. The overall granite size should overlap the top of the cabinets by 1 "to 1 1/2". The outside edge of the counter top should have a finished edge specified. A bull-nose edge is one style used often. Granite can be ordered in a modular system which is best for doing it yourself. With the weight of the granite it requires a 3/4 "plywood base over the top of the cabinets for support.
Choosing granite includes selecting grade, color, layout, style, etc. Once the choices are made the granite must be ordered as soon as possible. It can take a couple to several weeks for delivery. If you choose to use a granite fabricator, the fabricator will usually want to do its own measurement and installation to be sure the work is done properly. Lastly, make sure there is proper storage space for the slabs before they are installed.
The top of all the base cabinets must be covered with the 3/4 "plywood to support the granite. The plywood should be attached with screws.
The granite slabs can be very heavy. Get the help you need and handle them with care. To cut (scribe) to fit, apply duct tape to the base of the saw to protect the granite. Use a dry cut segmented diamond blade on a circular saw to do the necessary cutting. Test the fitting for pieces that are cut.
Place the slab in place on the sink counter. Trace around the opening exactly for the sink onto the plywood. Remove the dry-fitted slabs only. Make a pilot hole in the plywood then use a jigsaw to cut the sink hole. Cut about 1/8 "outside the line you drew for the sink.
Leveling the slabs to join the seams, use a 1-1 / 4 "screw from underneath to raise and lower the countertop.
To glue the counter top down, having others helping, lift the granite and put half-dollar dolops of silicone all around the perimeter of the cabinets. Spacing should be about every 6 "to 12". Next place a bead of caulk around the sink rim sealing the sink to the plywood. Then apply a second bead on top of sink rim. This will waterproof the gap between the sink and the counter top. Now gently lower the granite slab back down.
To glue the seams tape over each seam with blue masking tape. To fill in the seams start with a dollop of polyester-based resin and add a little amount of color. Try to match the granite color. Mix the resin with a putty knife checking the color match. Make a few different colors to blend the seam. Add the hardener 3% to 97% resin. Mix only small amount for the 5 minutes before it will dry.
Apply a base-coat of a neutral color mix to the seam. Smooth as applied also adding a little lighter color. Put in the darkest color last. As soon as finished remove the masking tape. After the resin is washed smooth it out with a seam stone using a slow speed keeping the stone moving in small circles.
One item I often think is that this project is not the same as glass table top replacement.