When I was laying bricks for a living, we made our mortar at 5:1, 5sand to 1cement. That's too much for this though, I made my "hills" at 15:1. Your crawler is all that going on it, it does'nt need to be person load bearing really.
As far as mixing it, a wheelbarrow is best, failing that a bucket. With a barrow, use a garden hoe to mix it up dry first, move it all to the front of the barrow, add water (about 7ltrs if mix is 15:1) then start chopping at the dry mix, slowly adding your dry mix into the water and mixing it as you go. Don't just try mix it all in one go, it'll end up a big blobby mess. Much easier to do it slowly and evenly. Another trick, add some dishwashing detergent to the water. It acts as a binding agent. If the sand you use is too coarse, the cement and sand will not mix properly and the water will just rise to the top and the solid stuff (sand+cement) will sink to the bottom. Makes it much easier to work with. Mix it to the consistency of mashed potatoes, so you can work with it on the hill and shape it better. Too wet and it won't stay.
Throw in some small rocks with your mix. Bigger rocks can go on later and set in with more mortar. Any left over mortar, add some water to make a wet slurry and slop it on any slick rock faces for more traction.
If your build area is prone to weeds, kill those buggers now. The first area I built has weeds growing thru the cracks which need spraying a lot. Might also help to leave some sort of access hole to man-made hills for spraying weeds under the hill if needed.
I'm gunna get around to posting up some pics oneof these days of my work in progress:roll: