I want to make 3d shapes of my own making to stick onto a canvas as a base for a mood board. Does anyone know what is the best way of going about it?
One option you have, considering that one of your tags is papier-mache, would be to cut cardboard of suitable thickness into shapes which could be stacked to form the contour you require.
A resource directly related to this concept is software known as Flat Fab. Even though the software is directed to laser cutting of cardboard and other flat material, one can use it to create prints that can be managed in a manual manner (craft knife, scissors, etc.) if access to a laser cutter is not possible.
Even though this particular image may not fit your requirements, it shows segments of the results of using the software.
Your question is tagged with adhesives, yet has no question related to that tag. That would be best addressed in a separate post, if you require adhesive related information.
Another option would be to use molding clay to form your shapes. It would require to create a scaffold or armature within the clay that extends to the back of the formed clay, allowing for a surface on which to apply adhesive to bond to the canvas.
It is probably easier to form clay than to cut cardboard, unless you have a laser cutter and you may have greater flexibility with clay.