Dig them the way I did mine at home, it,s hard work but you only do it once!!
First decide the size , the idea is that you can work on them from all four sides without walking on them.
I make mine to suit B+Q gravel boards which are 2.4 mtrs long (8 ft) and 1.2 mtrs wide (4ft).
Mark out the area and dig as deep as deep as you can, remove the soil and place to one side.
Line the base with double thickness membrane or Geotex, fit the frame and line the sides again with membrane or Geotex. Tack or staple the side lining to the framework.
Now thew hard work!! Seive the soil, I use a homemade seive made with 20mm weldmesh and cover the base with three good barrowloads. Follow these with compost or manure to the same quantity.
Repeat until within 100mm of the frame top. Using a fork , mix by "corkscrewing" vertically. Finish with seived soil to as great a height above the frame as you can manage. If you wish you can mix pelleted
chicken or farmyard manure into this layer.
Water it well!!
Leave for a week to settle (it will) then rake and sow/plant.
Doing it this way means that for three years you do not need to do any thing but weed and feed.
If like most of us you can,t leave it alone, you can only improve the bed by adding more and more organic material.
Because you have the depth of cultivation crops can be sown and grown at much closer spacings.Carrots are sown very close (I allow about 40mm between seeds, and take my time doing it!!)
Parsnips, Onions,Leeks,Beetroot,Shallots,Garlic all perform well in a bed of this type. Brassicas also do well but I,ve found that the Italian varieties do best here ( N Wales), they do however grow tall.
Beds this size are also easy to protect with nets against Cabbage White, I use fleece to protect against Carrot Fly.
You really can,t go wrong with this type of growing as long as you get the depth in the beds,mine are all about 600mm deep (2ft) and it,s really worth the effort to do it right from the start.
Once prepared, weeds pull out far more quicker than your crops will !!
Hope this helps.