There are two general methods of slurry preparation. You can mix it thin like a milkshake and it will naturally spread itself thin (as seen in the photo to the right). The slurry layer in between the earth blocks will only end up being about 1/4" thick. The pros of this method are that it's faster and you use less slurry (so less preparation); it's also thought to create a stronger bond than dryer slurry. The con is that it's a little more difficult to keep your earth block course perfectly straight; this is only a problem aesthetically, as you can still keep your wall perfectly straight and level on the vertical line...just not horizontally...i.e. you will see undulations in the courses when viewed from the side. If you are going to cover your walls with a plaster, stucco, etc. then the aesthetics don't matter anyway, and you might as well use the faster thin-slurry method. On this barn project, we used thin slurry because we were going for a rustic look.
The alternative method is to make the earth slurry thicker as you would with a traditional mortar. In this scenario, you end up applying and working the slurry with a trowel (as opposed to pouring it on with a shovel), and you end up with about a 1/2" slurry layer in between the earth blocks. The pro is that you can manipulate the amount of slurry you put down for each earth block and end up with a perfectly level line of earth blocks on each course...which is more pleasing to the eye. The cons are that you have to prepare twice as much slurry material and troweling the material onto the wall takes about twice as long as pouring it onto the wall with a shovel or scoop. This method is only preferred if you are going to leave bare earth blocks showing at the completion of your project. For a visual reference, the thick slurry method was used on the home we built for the show "Building Off The Grid". https://aectearthblock.com/building-off-the-grid