Gathering Materials
First, you need a Guardian Crystal Drop, which only appears when you kill a Guardian with a player-dealt final blow. Farm Guardians in an Ocean Monument or use a Guardian spawner farm. You'll also need standard Create Mod components to craft the turret itself.
Placement and Power
Place your 
Basic Guardian Beam Turret on a solid block where it has a clear view of approaching mobs. Connect a Create Mod power source (like a windmill, water wheel, or motor) to the turret using shafts or other transmission blocks. The turret will automatically lock onto any hostile mob within 17 blocks, provided there's an unobstructed line of sight.
Optimizing Performance
Remember that beam charge-up speed depends on rotation speed (RPM). Faster rotation means quicker attacks. Experiment with your power source's speed to find the balance between energy efficiency and defense responsiveness. The turret will handle single targets well, but consider upgrading to an Advanced variant if you need to defend against mob swarms.