Without sounding dismissive, exactly as it sounds. Sleepers, or well supported treated timber, ensure the ground on which it is stood is free of perennial weeds, especially things like bindweed, horsetail, then fill it up. I'd use good locally dug topsoil and 'improve' it with some well composted farmyard manure. Depending on what you want to use it for, choose the correct soil mix.

don't forget to peg the side wood down or they may collapse under the weight of soil. especially after heavy rain.

A raised bed does not need to be made from wood Bulba has made several in our garden from stones as they were for alpine plants... What do you plan to put into your raised bed(s)?

Raised beds can be made from most materials, they are easily made with tanalized gravel boards and 2''x 2'' wood suitable sawn stakes into lengths 2' or so long, holes dug the stakes sunk into them and concreted in, then whilst wet squared up with a spirit level left for concrete to harden and when it is hardened the boards screwed to these stakes.The framework now complete soil can be added. Total cost approx ten quid per each 6'6'' sq bed..