With trowel, comb thinset onto backs of side pieces.
Set each piece in place on top of hearth and press against brick. Make sure inside edges slightly overlap the sides of the firebox.
Lay straightedge across tops of side pieces. Check that they align. If not, pivot slightly.
Butter thinset on top piece. Set it on the side pieces (as shown). Press tight to brick. Check that joints are flush.
Allow thinset to harden overnight.