You can either make it out of wood or cardboard i like cardboard because it can be more detailed,though making a gun out of cardboard is mostly improvisation so there isnt really any specific way to make one becase you might make it hollow or you can glue a bunch of cutouts together.
Step 1: Things you need
a table saw or jig saw
a copper tube-should be about an inch longer that you want the barrel to be
help from an adult
a picture of the gun you are trying to make
a glue gun with glue stick cartridge things
paint-spraypaint and maybe some acrylic paint
thin cardboard-i used the cardboard from a cereal box
these are the basic things you need but ill mention some other things you can add and how i added them.