It is a lot easier to darken cabinets than lighten them. The preparation is the same as <a href="https://www.house-painting-info.com/refinishing-kitchen-cabinets.html">Refinishing Kitchen Cabinets</a>.
The major difference is the stain used over your existing finished kitchen cabinets. I use Old Masters stains when darkening cabinets. Their wiping stain or gel stain will do the trick depending on the final effect desired. The gel stain can be darker or more rich in color.
Multiple coats of either type of stain can be applied for a more dramatic look. Make sure to allow 24 hours between coats. Complete drying of the stain between coats is very important. The second coat could loosen the first if it isn't completely dry.
Once the stain has fully dried a clear finish can be applied. Any good user friendly wood finish will work. Spar varnish or acrylic urethane will work great depending on the level of hardness desired. The varnish is harder when fully cured.
Check out <a href="https://www.house-painting-info.com/restain-cabinets.html">Restaining Cabinets for a New Look</a> for more info.