Injections
Injections hold spinal fluid, and when one is injected into a living mob, it turns it into a Pure Titan. Some qualities of the source fluid is transferred into the Pure, e.g. Attack Titan spinal fluid will make stronger Pures, Colossal Titan spinal fluid will make bigger Pures, and so on.

Spinal Fluid
Spinal fluid can be taken from any transformed titan by simply right-clicking them with an empty injection. If you yourself are transformed (not as a Pure) and want to take your own spinal fluid, you can shift-right click an empty injection. To transform something, just right-click it with a filled injection. You can also use an injection on yourself by shift-right clicking it (be careful!!!).
Obtaining

Commands
You can use the command /function phantom:give/injection/<type> to give yourself an injection of the specified type.