Select from Inch or Metric. This is determined by the part (piston, cylinder etc) you will be using the seals with. If your part's dimensions are specified in mm, your seals should be too!
Choose the right seal type for your application. If you know the general category of seal you need (for example, piston u-seal or wiper seal) just click on the category name. If you don't know the category, visualize your seal sliced in half to determine the profile, and choose the group that is most similar to yours.
Narrow down the selection. Compare the profile of your seal to the pictures, select the closest match.
Choose the exact seal. Select filters on the left to find the exact seal that fits your piston.