Work out how much you want to raise the chair. The best way to do this is to sit down and estimate where a comfortable position would be.
Decide what kind of raisers you want. If the chair needs to be raised a few millimetres then felt stoppers glued to the chair leg bottoms will be fine, if you need a few centimetres then plastic raisers will be needed.
Purchase the appropriate raisers. These are widely available and come in a variety of styles to suit your dining room set.
Fit the raisers to the bottom of all four legs. The plastic versions fit on over the leg itself, the felt stoppers are self adhesive.