Oil the bottom and sides of each party tray. I didn't need to go into all the nooks and crannies.
While wearing long gloves and eye protection, pour some powdered concrete mix into a 5 gallon bucket.
Add some water, then mix with a small garden shovel until you get a smooth porridge consistency. I found mixing small batches at a time easier to manage. And remember to rinse your tools frequently.
Set the party trays on cardboard for extra protection. Pour concrete into each tray until it's at least 2-3" thick. I started filling the trays with a shovel, but as the bucket lightened up, I poured the concrete direct from the bucket.
Lift the trays while filling, then drop them so the concrete gets into all the groves of the tray shapes.
End with a smooth trowel across the top, then allow to dry overnight.
Removing the concrete from the trays
With cardboard underneath, place one hand across the top of the concrete, then pull the tray away with your other hand until it pops off.
Gently set the stone on the cardboard and allow to cure for a day or so to be on the safe side.
To position the garden stepping stones
Level the soil, then position a stone where desired. Twist it abit to embed it into the soil.
Place a construction level across the top, then continue to add more soil underneath until the stone sits level.
Protect the concrete with a topcoat if desired. However I loved mine as-is!