The exact method of setting up a flash diffuser depends on what kind of diffuser you are using. In the case of the large dome light, you will need to put the diffuser in front of the dome light. Usually, such diffusers will have Velcro tape around the circumference and fixing it on the light fixture is not that difficult. It is even easier if it is a diffuser for an on-camera flash unit. Most diffusers will easily slide over the flash unit and stay in place. Some of them will have a Velcro or elastic strap that you can use to fasten the diffuser around the flash.