I don't have a lot of time today, but I want to get my seeds started by the weekend.
For making the micro-blocks (which are approximately 3/4-inch cubes), it's recommended that the mix be sifted through a 1/4-inch screen. This is to remove any pieces of material large enough to keep the little blocks from compacting properly. (If they don't compress correctly, it seems that they'll more easily fall apart.) Fortunately, I have both 1/4-inch and 1/2-inch sieves for my soil sifter.
It's a nice day, the warmest we've had so far this year (76 degrees at 4:21 PM), but it's windy--10 to 15 mph from the south, gusting to 30. The potting shed is 75 feet west and hasn't been made ready for the season yet, so I set up a "sifting station" up close to the southwest corner of the house. The garage helps break the wind.
I want 900 of those little blocks. The math says that, allowing for compacting by 2-1/2 to 3 times, I'll need between 1/2 to 2/3 cubic foot of mix. Since the unused stuff can always go back in the bag, I'll go for about a cubic foot. If I fill one of the green tubs about 7 to 8 inches deep, that should be enough.
Sifting takes less time than setting up to do it. I use the plastic coffee container as a scoop.
The Pro-Mix has a very fine texture. The only thing left behind on the screen are clumps, soft little clods up to about an inch in diameter, and they break up easily into very fine mix, but it's faster just to shake the sifter and dump the clods in the other tub before returning them to the bag. With this mix, at least, the sifting step can probably be skipped.
Adding water and playing in the mud to make the micro-blocks!