All you need to build them are some lengths of pressure treated \/ tannalised wood and some wooden stakes. Of course you can use other materials, such as old \/ new railway sleepers, brick or stone, logs, plastic raised beds are also available to buy. In fact nothing solid enough and safe, avoid any substances that may contain toxic compounds, or with sharp edges. Just cut the lengths of wood to the size of bed needed, recall a 4 to 5ft wide bed allows to get easy care without needing to step on the ground which might cause compaction. Mark out and dig a narrow trench about 2 inches deep for the planks to sit in on their borders, then support the plank with stakes driven in to the earth every 3-4ft.
For additional stability it can help to have stakes on both the inside and outside edges and maybe nail or screw the boards to one another where they meet in the corners. Ensure the boards are good and perpendicular as you drive in the stakes. The stakes should be thrown in and sit 1 -2 inches below the top border of the plank. Sometimes it can help to line the inside border of the planks with a non permeable liner, or plastic. This keeps the damp ground off the wood and lengthens the life span of the boards a little.
Once constructed the raised bed may be full of good quality soil which has been improved with organic matter like well rotted farm manure or compost. Spread the soil consistently making certain there are no air pockets remain at the edges and corners. Fill the bed to the top of the boards, as it’ll, in time, settle and the degree may sink. Paths between raised beds must be lots wide enough to walk along and ideally wide enough for a wheel barrow. Between raised beds I like to put down a weed membrane on the dirt surface and put a 2 inches deep layer of mulch.