The best way i've found to deal with couch grass is a two pronged approch, firstly spraying with a glyphosate based weedkiller, see back of pack for active ingredient, but buying it marketed as glyphosate rather than roundup/weedeol and the like would be much cheaper. Glyphosate breaks down on contact with soil, so wont harm the soil. After about three weeks, all the top growth will be well and truly dead, and the roots should also have been largely killed off, i then turn the soil and remove and roots I can see. After fertilising and planting, its just general spot weeding. If you intend to grow in raised beds, depending on the depth of the beds, you'd get away with laying a woven landscape fabric eg: phormisol over the couch grass after applying glyphosate then building raised beds on top, this would deter any regrowth.