when using latex moulds to make concrete ornaments use a mix of  four of sharp sand to one of cement use warm water to mix up the concrete as this helps reduce the curing time of the mix you can also use a liquid rapid hardener and frost proofer in with the water as this speeds up the drying time and protects the ornament out side in the winter from cracking ,another hint is demoulding if the mould is hard to get off smear washing up liquid over the mould and this helps with the mould to come away smoother with out the latex sticking to its self . another trick when making stepping stones or slabs is to add a hand full of 10mm pea shingle to the mix this makes the finished mould  stronger when walking on them  and prevents them from cracking or snapping and if you leave them in direct sunshine to cure or under cover ,make all your stepping stones/slabs the same or there will be a slight colour difference in the finished item when placed together.