You must tribute summon a Steelswarm monster while this card is face-up to activate effect.
Steelswarm monsters are made up of 2 kinds of monsters, the low leveled monsters that should be used as tribute fodder, and the powerful high-leveled monsters who gain effects when you use Steelswarm monsters to tribute summon them. Additionally, when tributed Steelswarm Caller and Steelswarm Gatekeeper allows you to swarm the field with Steelswarm monsters.
Here are the monsters you should use as tribute fodder:
You can use Advance Force to tribute summon Steelswarm Longhorn by tributing only 1 Steelswarm Mantis. Since this is a tribute summon, you still gain additional summons from Steelswarm Gatekeeper. Use Advance Zone and Trial and Tribulation to gain various effects based on the number of monsters you tributed for that turn.