It can be done, but I'm not aware of the "easy" way. Just about everything that has to be done in this project is infinitely easier IF you have a helper, expecially one that is mechanically inclined.
You will need a funnel to get the concrete into the holes in the cinder blocks. Naturally, the small end should fit into the openings in the cinder blocks, and the large end should be larger than the shovel you will use to transfer the concrete.
If you cannot find such a funnel you will need to make a cone-shaped funnel out of some thin sheet metal [like galvanized sheet metal; I usually use galvanized "flashing" material] which is slightly smaller at the small end than the openings in the cinder blocks. The top should be slightly larger than the width of a shovel which you'll use to transfer the concrete.
Before trying to form the sheet metal into a funnel, I suggest you practice the task using a sheet of paper, many times until you get moves down "pat."
To form the cone funnel, take a square piece of flat sheet of metal with your hands on opposite sides. Wear substantial work gloves to avoid cutting your hands to ribbons. Curl the edges in at an angle to form a cone, smaller at one end and very wide at the other.
Once you have the proper shape, have an assistant secure the shape at each end with "C-clamps." When shaped as you want it, and secured, use a drill to drill a line of holes sized for "Pop Rivets" [1/8th inch diameter by 1/8th inch long should do the job].
Another option would be to make a tapered chute ["trough"] by again using thin sheet metal. Bend two opposite edges upward 90 degrees and not parallel so that it is narrow [smaller than the holes in the cinder block] at one end to create the sides, and at the other end, wider than the shovel you'll be using to transfer the concrete.
Once you have a funnel or chute capable of funneling the concrete into the holes, you will need to use a concrete with SMALL AGGRAGATE [the stones or rocks in concrete]. IF the aggrate is too large, it will be very difficult, if not impossible, to get the concrete all the way to the bottom of the staggared channels in the cinder block, and there is also a tendency for voids to develop.
Once ready, with the assistance of a helper, place 5 or 6 shovels full of concrete into a hole, and then use a long rod which will easily fit into the opening. I use a piece of #3 "Rebar" [round steel reinforcing bar, 3/8th inch diameter] to "rod" the concrete, and rod it a half dozen or so times.
Rodding is done by plunging the rod down into and up out of the poured concrete to vibrate or "jiggle" it around to make sure there are no voids which weaken the cured concrete.
Then repeat by adding a little more concrete and rodding it, deeply enough to penetrate the joint between the current pour and the previous one. Repeat this procedure until a single void is filled. Do ONE void at a time so as to keep a "wet interface" between the already poured concrete and the concrete you add. This results in what is called a "monolithic" pour which provides the strongest cured mass.
IF you don't do this, and allow the poured concrete to set, or the surface to dry, and then pour fresh concrete onto the old surface you will get what is called a "cold" joint which will be much weaker than a monolithic pour.
Once the first columnar void has been filled, then do the second, and so on, being careful to avoid any cold joints. For maximum strength, you could pour the concrete around a #3 rebar installed and left in each channel.