Best way for levelling slabs?

[spader]

I am putting down some slabs in the back garden by myself.
I dug out the old plants and trees, dug the ground 15 - 20cm, and put down type 1 MOT sub base.
I ordered sharp sand and cement. In the past, I put down sharp sand on the compacted MOT type 1, and pour the cement, mix them with rake, spreading out to the area for slabs. I would get the builders trowel, and flatten the sharp sand and cement mix, and check the level with the bubble. Slabs would put down on it, with further adjustment adding more sand for levelling. This method worked ok in the past. But I never got total flat level from the slabs laid.

Is there better way trying to achieve total flatness in slab laying?

 

[DAZB]

To get a level or flat bed over a wide area i have always use rails which involves setting in either a length of metal conduit or even a scaff tube a certain distance apart and level them up (if level is what you need or create the fall you want) and then pull a straight edge flat over them to create the bed. You then remove the conduit / scaff tube and fill in the trench it left flush with the surface and that way you have a flat bed to lay flags on.
For example if you have a 9 metre width lay a pole at each side and one in the centre then lay and level your base from these poles and then slide them down to set out another run and repeat until done.