My first allotment: neglected and full of tussocky grass, nettles, bindweed, couch, creeping buttercup etc. I'm a fan of little by little digging and weeding and planting as I go, while strimming the rest of the plot to keep it flat. I put the perennial weeds into sacks to rot down and compost the rest. It takes a long time to get round the whole plot but it's very encouraging to see beds of veg springing up behind you. It is so productive now that we can't keep up with it.
My second allotment: started in late May in a tussocky grass field. I'm experimenting with two methods - the steady digging and planting as above (soon to contain leeks), and lasagna beds. These are layers of wet newspaper, wet cardboard, wilted comfrey, turves from the digging area, manure, compost, soil (from first allotment) and anything else I can find. Again, I am filling them with plants as I go (tomatoes, cucumbers, squashes and corn so far). In my case the beds are long and narrow, with paths made of cardboard and woodshavings. Time will tell whether or not the grass will come up through the beds and paths, but devotees of the method seem to be very enthusiastic, and the results so far are much faster than the digging.