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Your trident needs to go back outside ASAP.  This is the wrong time of year to work tridents (and most decidous trees, really).  You need to find a place outside, protected from wind and sun, let the tree go dormant and spend the winter  cold, sometimes frozen, but asleep until next spring.

Under what conditions do I bring it inside?

Bring it inside to work on it, then, back out it goes.

Please be aware that trident maples are a little more sensitive to the cold than other maples.  The top parts are ok with the cold, but the roots don't do well if they get really hard frozen.  Set the pot on the ground, and mulch it in really good.  Some I know use a type of heating pad to keep the ground a little warmer for their tridents.  Again, your local bonsai club should be able to help you. 

Bill Valvanis, in Rochester, NY can guide you.

I heard of that place. Unfortunately, it is 6 hours away from me. I live in the city. Brooklyn Botanical Gardens have a beautiful collection. I'm trying to find out if there is some kind of workshops. Otherwise my local sources are extremely limited. 

Bring it inside to work on it, then, back out it goes.

Please be aware that trident maples are a little more sensitive to the cold than other maples.  The top parts are ok with the cold, but the roots don't do well if they get really hard frozen.  Set the pot on the ground, and mulch it in really good.  Some I know use a type of heating pad to keep the ground a little warmer for their tridents.  Again, your local bonsai club should be able to help you. 

Bill Valvanis, in Rochester, NY can guide you.

What kind of mulch should I buy? how to and where to layer it?

I had a create idea of building a mini green house for my satsuki azaleas and trident maple. Once I built it, I will place it on my balcony.