I have the same file somewhere too.........there's one that's 'done the rounds'
Basic recipe is cement, crushed oyster shell and rock salt. Mix it up and place in a mould made in a sand pit. Allow to set then place in a curing tank and do loads of water changes till all the rock salt has dissolved and the water parameters are neutral. Then pop it in your tank and wait for it to cure
A great tip for making small rocks is to put them in your toilet cistern when set as this flushes the rock with clean water everytime you flush (does mean your toilet loses a little flushing volume however) and speeds the process up massively.