One way is to take reference pictures of real world or in-universe weapons, scale (based on listed dimensions) and trace them and then print out templates. I'm pretty sure there are free programs out there that you can do all this with. Check out Punished Props on YouTube, they have several tutorials about modeling and scratchbuilding props from templates made with low cost/free software.
From there, it's transferring the templates onto material, cut/shape/carve/sand paint.
You can also trace the shapes of parts you like from Nerf or other toy weapons onto paper, cut them out and assemble them into new blaster-like shapes, transfer to wood or PVC board and then cut/shape/carve/sand paint.
Use PVC pipes as barrels and scopes and matching fittings to add dimension. If you use PVC trim boards (like sintra but thicker and cheap) then you can do a lot of the assembly with pipe cement - wear your respirator and ventilate your workspace.
I really like
scratchbuilding weapons. All of mine start as a 2D template that I've drawn (or borrowed).