What sort of size are they and what sort of things do you want to grow? In general, mine have about 50% topsoil, 25% well rotted manure and 25% miscellaneous compost, some home made, some commercial stuff, often from pots at the end of the season. The good stuff comes from the topsoil and manure, the rest is really to improve structure.
A trowel in the hand is worth a thousand lost under a bush.
That's about 2.4 cubic metres (say 2 as you won't want to fill them to the top.) I'd get one builders (tonne) bag of topsoil (=1000 litres), ten 50 litre bags of well-rotted manure (unless you can source half a tonne locally) and five 100 litre bags of any cheap compost (if you don't have any home made stuff.) Most veg will grow really well in that except acid-loving plants. Top up every year with 2-3 inches of well-rotted manure and you won't even have to dig it.
A trowel in the hand is worth a thousand lost under a bush.
I placed stones at the bottom for drainage and then alternate layers of top soil and manure as I thought that the roots would grom down to find the manure and the water held in it. Obviously no way to check if it worked
Everyone is just trying to be Happy.....So lets help Them.
The base layer: I would defo dig in (spade depth) a good quality soil improver. Laborious if whatever is under is unyielding, but it really does help (In my opinion). Then fill as above. My neighbour has created two raised beds on top of extremely hard clay. He has just added a mixture of top soil and Easy gro multi purpose to about 6" depth - but the base clay layer remains intact! Interesting!
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I placed stones at the bottom for drainage and then alternate layers of top soil and manure as I thought that the roots would grom down to find the manure and the water held in it. Obviously no way to check if it worked