Consider how and where you plan to display the canvas after it is painted. If you plan to use it on a frame, stretch the canvas over the frame and size it before painting. After paint or glue is applied to canvas, it can shrink and tighten up.
Prime the blind for painting. Cover with a coat of acrylic gesso (the white "primer" used to prepare raw canvases). If your painting will be more whimsical than professional, you also can do this by applying a couple of coats of latex base coat to the blind, enough to soak through and make a solid surface for painting.
Put a latex base on the blind if the old blind is a dark green color and dark green is not in the color palette you had planned.
Attach the canvas blind to the frame if the blind already has been treated with a coating to stiffen it. There's a chance old blinds may crack around the edges as they are bent around the frame. Use a fabric glue to attach old blinds only to the front of the frame, if cracking seems likely to be a problem. Paint the sides of the frame with base coat.