How do you use synergies?

[a pimp named helldiver]

Is it like an automatic thing that applies to your guns? Do you need to combine them yourself? I'm confused.

[Unsanitary]

Mostly automatic. If you have one you will get a blue arrow above your head. To tell what syngerize you need to open your book and hover over the current item.
Some items like the megahand have different modes regarding synergy. To access these you must reload while at full ammo

[MechaTails]

Check the Gungeon wiki, a lot of the synergies are listed there on each item's page.

[blue sphere]

Most of the time you'll know if you got a Synergy because once you get the two items that synergize with each other the game will come up with the name of the synergy you just got.