buying washed sand and washed gravel is cheap or you can buy washed aggregate mix. Most expensive and easiest is the just add water mix in the bags. Incidentally you're mixing concrete not mixing cement
. If you go to Burnco to buy bags of cement you have to ask for "Portland cement" because believe it or not they don't know what "cement" is....I got in a huge fight there last year trying to buy bags of cement .....anyway I digress. Mixing in a wheel barrow is ok for smaller quantities and works best with a hoe. Not the kind of hoe you pick up at a cheap bar...a gardening hoe. Although a good hoe with a strong hoe might be a good combination....I digress again.
I do 3 shovels of aggregate and one shovel of cement, mix it dry until it's well blended than add the water. If it's "self levelling" you added too much water.
Sounds like a lot of work, I'm glad my projects are mostly over.