Most garage slabs only receive a minimum of sealer when originally poured. So years of storage and spills really take their toll. Plus the natural porosity and chalkiness of concrete makes it almost impossible to keep really clean.
Check to see if there is still top sealer on the floor. Drip water onto the slab. If the water beads up, there is an existing sealer that may interfere with the adhesion of the epoxy. Depending on how much sealer is left, you may have to repeat the etching process.
Take time to fill concrete cracks and voids as these imperfections will show through the epoxy.
Mix (1) part A to (2) parts B on a clean piece of cardboard and then fill cracks and voids using a plastic spackle knife.
Allow the filler to harden for eight hours and then feather down any edges with a coarse sanding block.
Give the space one more good cleaning before applying the epoxy. Be sure to get close to edges and walls for bits of block and concrete that will get into the finish.
To make cutting in easier, apply painter’s tape along any block and wall plates (basically anything you don't want the epoxy on).
Your epoxy kit should have come with a large and small can. The large can of Part B may feel light but it is only partially full so that you can pour Part A into it. Epoxy is available in various colors but the two most popular are gray and tan.
Pour Part A into Part B.
Then mix thoroughly until the liquid is homogenous.
Start rolling the epoxy out in 4' x 4' sections. Keeping the wet edge, lightly broadcast the surface flakes into the wet epoxy. Also be sure to plan your exit strategy as the floor will need to dry for 24 hours before light traffic. You don't want to paint yourself into a proverbial corner!