The first step to refinishing your existing cabinets is to remove as much hardware from your cabinets as possible. It will be helpful later if you can come up with a system to remember which piece of hardware goes to which cabinet. Also remove the doors and drawer fronts before starting. Sand all existing paint, stain, or finish off your cabinet boxes, drawer fronts, and doors. Also, sand off any knicks or dings from over the years to create a smooth surface. Once you start staining kitchen cabinets, a mark you missed while sanding will jump out at you. Try and do a good job of sanding.
Second place a drop cloth or cardboard on anything you do not want the stain on. Lacquer thinner or acetone you may be able to use to erase the stain but does not always work and is not recommended on all surfaces. Apply the stain to a portion on the interior to test the color, allow it to dry before continuing.
Third if you are satisfied with the tested color you can start the staining process. Wipe the stain on the surface in broad strokes. Depending on the age of the wood you may need to apply more than one coat. Newer wood will soak up a lot of stain. Aging or previously stained wood will soak up very little stain. An extra coat will also help legthen the time needed before they have to be stained again. Let the stain dry for the time recommended on the product.
Finally when the stain is all dry you can reinstall any hardware you took off your cabinets. Then reinstall your doors and drawer fronts to the cabinet boxes.