Chop the rhubarb into small pieces, place them in a freezer bag, flatten the bag a bit, and freeze it. Martha Stewart (the link opens in a new tab) would advise you to lay the pieces on baking trays lined with baking paper, freeze them for several hours, and transfer them to freezer bags once solid; this way, the slices will not stick together when frozen. However, I rarely bother. For more details on this subject, check How to Freeze Rhubarb.