Welcome to the forum jkm.
You should start with a hot wood fire, get it burning with lots of red coals.. Then add a 1-2" layer of anthracite coal, In those inserts I'm pretty sure you are burning Nut size coal.
Close the front door, and keep the ash door open for lots of air under the fire, once the first layer of Anth. coal is burning well, add another layer , again keeping the front loading door closed, and feed lots of air under the fire.
Once you have several inches of hot anthracite coal burning, close your ash door, and open the air control on the ash door a few turns from closed. Fill the hopper with coal. It will cover the existing fire and look like it may smother it, but soon you will have thin blue-white flames coming up through the coal and soon the whole bed of coal will be red.
Turn down the air control on the ash door to control heat and burn rate. The correct setting for the air control varies with the height and draw of your chimney, how cold it is outside, and how hot you want your fire.
There are several Hitzer insert owners on the forum, I'm sure they will post as well and correct me if any of the above is wrong.