Using your power:

Can be used at any moment in time. Some have restraints, if yours has a restraint abide by it. But in multiple chains can be used. Can also create a chain and use it solo and delete chain after I've seen it. 

Evolving your power:

Powers evolve the more you use it. For now, only one person has evolved their power because they have used it. 

Fighting with your power:

There is no formula. Use your power and state what it does. Then give the other person a turn to reflect etc. Eg. 

Bla bla caused icicles to appear, ensnaring the others foot. 

The other used fire to melt the ice before lashing out with flames towards their face

Bla bla could not avoid it, causing...

Be realistic. Do not make it OP. There are a few OP characters, you will personally know if your character is OP. If you make your character OP and they're not. I will personally intervene in the fight.