Render Sprite on Screen at 3d game

Hello, everyone! I have an image to draw at screen, but I need to draw only a part of it: the area displayed depends on some parameters (yes, this is smth like a progress bar). That means I need to use self-written simple shader. I created a SpriteRenderer, assigned it a sprite and a material. There I completely lost: I don't understand, how to set a rectangle to draw the image. Should I assign a geometry to material or something else? And how to map the coordinates assigned to screen ones? I have no idea, please, help me!