Seems that you are making the vehicle for the first time.
It is basically a simple process. You will have to make steel clamps of say 3mm 0r 5mm steel sheet. Then you weld it to the roll cage in pairs so that the suspension (wishbones & dampers) fit there.
Similarly for brakes, engine/motor and every other installations, you will have to use different kinds of mountings and that can be based on the design & mounting points of the part.
I would suggest you to carefully examine the joints of some existing baja vehicles. See the different kinds of mountings on the pics available on internet. If you can see a real baja vehicle, that would be even better. Then only you will understand these details.
CORELOGS