Note: with the introduction of conduit jumps in June 2021, things have changed. Conduit jumps ignore ship mass, so if you're conduit jumping, keep your plates online. If you're doing regular bridges, offline them.

To offline your plates in a station or structure, use the Fitting Window. In space, use the HUD.

Via Fitting Window: Hit Alt+F and either click the module’s power button or right click the module and select Put Offline:

Via HUD: Right-click the module and select Put Offline:

If you jumped into combat, online your plates before activating any modules (you need 95% cap to online one module).

If you don’t have passive modules showing, fix that now (you should do this anyway to keep an eye on module damage when managing heat). Left-click the hamburger menu at the bottom-right of the HUD and check Display Passive Modules:

For more BLOPS-related stuff: jambeeno.com/blops