Prepare the materials.
After being pressed the chaff will be very compacted and will need to be separated by hand into small fluffy(ish) chunks before being mixed with the spawn.
CAUTION: It should be no surprise that 170F water is HOT.
Once the chaff has been broken up, we've been using a 4 parts chaff : 1 part grain spawn mixture. Dump the grain spawn directly onto the chaff. Be careful not to contaminate the grain spawn. Seriously, don't contaminate it.
I drilled air holes in these trays to allow oxygen to get to the bottom of the mix. Mycelium likes air but also likes humidity so don't let it get to dry.