Specification: Rubber band length: 3.5m Thickness of rubber band: 2mm Weight: 58g Diameter: 60-65mm Applicable venues: hard ground, concrete ground, training ground, etc. Trainer Instructions: 1. The trainer base is used with a rope tennis ball, first find the above items in the package 2. Find the black bungee cord and tie a section of the cord to the threading hole of the tennis ball, be sure to tie it firmly.The lanyard opening is in the circle at the top of the trainer and also needs to be tied. 3. Pull out the rubber cover on the surface of the trainer, fill it with water or sand, and then tighten the cover. 4. Find an open space, throw the tennis ball upwards, and then hit the tennis racket at a 45 angle from the bottom to the top. Package List: 1 Children's tennis rackets21inch 1 x Tennis Trainer Rebounder Ball with Rope 1 x Base