The basic mix ratios for cement, no matter what the use is
1 to 1.5 parts cement
3 parts sand
2 parts washed aggregate
by weight water should be .6 to .65 the weight of all other material in the mix. I think, I'm doing this from memory.
Additives: use acrylic fortifier for concrete it will increase strength. Buy acid resistant cement fiber, you won't be able to see it on a polished concrete surface, and it reduces chances of cracking.
Technically you can use just 1 part cement to 3 parts sand. Usually that is used as a slurry, or scratch coat, for refinishing surfaces or a top coat on a concrete countertop. If you use enough additives it will be a stable standalone countertop mix. There is a white cement you can use. For an aggregate look at washed pea gravel or crushed marble or crushed glass depending on what color you want in the countertop.
For sand you could try silica sand, but do be cautious going down this road. Otherwise there are colored sands of every color.
Most DIYers and professionals prefer precasting upsidedown in a form. That creates a smooth surface right out of the form when flipped back over. Build it out of wood, then spray it down with adhesive, and let the adhesive dry. Concrete won't stick to most dry adhesives, silicone, or polyurethane. I have a theory, that I haven't tried, that self leveling polyurethane concrete crack filler would make a wonderful form coating because it is self leveling and self smoothing. There will be some pinholes in the resulting cement no matter how smooth it comes out of the form. Fill them in with the before mentioned slurry before polishing. Some vibration may be needed to produce a smooth surface.
Renting a wet polisher for stone is highly recommended for finishing your concrete.
I also strongly recommend you pour multiple test squares, just 1 foot by 1 foot. You will make mistakes on your first attempts or want to change things.
Also consider building a rough tank around the form and putting your countertop under a couple inches of water once the cement has began to stiffen. It is the best way to produce a hard and strong concrete outcome.
Also take note that normal gravel used in cement is one part sand, two parts large rock. Just in case you find the recipe for foundation building cement.
Have done concrete window sills.