Acid staining of concrete is a simple process. Degrease any oil or grease stains. Rinse the floor to remove dirt. Spray, scrub, or brush the stain into the concrete. Let the stain dry. Rinse the floor to remove the residue. If you like the color, seal the concrete. If you want a darker or deeper color, apply additional stain, rinse the residue and seal. The decorative scoring can be done before or after depending on the effect desired. The concrete score can be narrow for a cut stone look or wide for a grouted tile effect. Decorative stenciling can be used to engrave a wide range of designs.
Depending on your level of experience and how comfortable you are working with chemicals, acid staining and scoring is well within most DIYers.

Old floors which have linoleum or adhesive backed vinyl may have residue which will need to be removed. There are several adhesive strippers on the market which do a good job of removing these residues. Most will require the use of a buffer with stripping pads. This is something you may want a janitorial contractor to do for you. If you have carpet down, where the carpet tacks are attached to the floor there will usually be divots and holes from the attaching nails. These can be patched with polymer cement. I recommend staining a border with a darker color to make these less obvious. However, as with cracks, some people like the distressed look. It all depends on the look you want and the final effect.
Acid colors are very dependent on the composition of the concrete. The color WILL vary. If you have a specific color you want, you may want to reconsider using acid stains. What you will have is a variation of shades. A black stain can vary from deep black to gray. Just be aware of this. Also, using solvent based sealers and waxes will make the stain darker. Water based sealers allow the color to remain somewhat the same as when dry.
If you have questions, feel free to contact me.

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When is a surface ready to be stained?
If the floor has a lot of construction residue, this will need to be removed. This can be old drywall mud or paint. If there is purple residue from the plumbing installation, I would recommend using a dark stain as these purple stains are difficult to impossible to remove. A light application of paint stripper with a low speed buffer and a black 3M pad works well. If there is old linoleum glue it will have to be scrapped and then adhesive stripper applied, scrubbed and rinsed.
After removing all residues a good rinsing and moping or a pressure wash is all that is required. Let the floor dry until all the moisture is gone.
Applying the Stain
The stain can be applied with either an all plastic pressure sprayer or scrubbed with an acid brush. It all depends on the effect you are looking for. The brushing tends to give lighter and darker colors with the spraying giving a more uniform color. As in painting, mask off any areas you dont want the stain to get on. Let the stain dry. Depending on your location this can take from one hour to overnight. Take a small area and remove the residue with water. Look at the color. If this is what you want, stop. If you are looking for a darker color, apply a second coat.
In addition to the color you want you can use different colors to highlight and blend in with the stains. Experiment for different colors. You can vary the dilution of the product for lighter and darker colors. Using brushes, you can cut in borders, do designs, highlight and contrast. Just keep in mind the stain colors will vary. If you know someone who has this done and you want that exact color, I am sorry but it will not happen. Do not set yourself up for disappointment. The decorative scoring can be done either before or after staining or during. It all depends on the look you want.

Sealing
If there is anything that will cause problems with your project, it will be the sealing. Too many people see the commercial floors that look five feet deep and want this look in their house. Remember, in these commercial buildings they have professional janitors using high speed buffers for these looks. If you are willing to go this route fine. Most people want these for the minimum of care they require. I recommend a good quality sealer and then basic wax for the maintenance. The ultimate is plain or tinted wax. While it does not have the five foot deep look, it is attractive easy to clean and maintain. I have given up on the shiny sealers because they wear, they are hard to maintain, and the failure of these systems is too high. A waxed floor will require periodic sweeping, occasional moping and when traffic patterns start on your floor, all you need to do is re-wax these traffic areas and you are good as new. The only drawback is the stripping and installing new wax which will be required. This can be once a year or once every two years depending on the traffic.
If you do decide to go with the shiny sealer, choose one which can be top coated with a maintenance product. The sealer itself will wear so maintenance will need to be done.