Here's the link for Part B: https://www.instructables.com/id/How-to-make-a-mushroom-log-from-Scratch-Bulking-u/
Here's the link for Part D: https://www.instructables.com/id/Part-B-Making-a-Mushroom-log-Transferring-Myceli/
Once the bag is almost entirely white it is time to bulk up the grain spawn. Make sure you transfer before the just before the grain is completely grown out, if the mycelium runs out of food it will slow down and possibly even switch to a vegetative state, not ideal for fruiting. If you only want to make a couple logs you can skip C and D, moving straight to Part E: Transferring Spawn onto fruiting substrate. One quart of spawn is enough for one 8"x 4" log.
For Part C you are performing the exact same procedure as Part A. The only difference is Quantity. Look at the table in the picture I included to help you decide on quantities. Make sure to keep the ratio of spawn to fresh grain correct. It should be 1/10 spawn to fresh grain. More is ok but any less spawn and it may not be able to take over before contaminants do. I usually move from the Quart/Liter Jar or Spawn Bag to the 8”x4” Spawn Bag at this stage.