Help with the making of knee pads

Hi!, I never made a mesh before, so it-s kinda difficult for me to create new items from scratch. I started making a volleyball set for the sims, until now I only made recolors, so no mesh involved. But now I want to make some kneepads, I made the basic shape (basically a rectangle) but I don't know how to proceed... here are the references, as well as the package and the blender file. feel free to edit as much as you want, I just don't know what to do know. If someone wants to edit it, please keep in mind that I want it to be maxis match, so no alpha or extra detailed textures, thank you!

Is also fine if you give me a step by step tutorial

Package + Blender file


(for the image right click and open in a new tab)