https://www.simplyscuba.com/collections/regulators-1st-and-2nd-stages
This video shows you how to easily and safely assemble your diving regulators for standard recreational configurations. It may look complex but with a few basic tools and a little time and patience any one is able to safely put regulators together.
Typically all you will need is an allen key to suit the port plugs on your first stage and a couple of different size spanners (an adjustable will be sufficient it that is all you have. Regulator designs typically fall into one of three first stage layouts which makes it much easier to follow this quick guide and ensure that your regulators are initially configured to standard recreational requirements.