Now that you’ve identified your plot, it is essential you clear the soil of any reoccurring and annual weeds before you plant anything into the ground. You will also need to clear the grass so that you have a nice soil bed consistency to plant into. The efficient thing to do is to clear the grass and put it upside down on the compost heap and a few months later it will breakdown and turn into a rich top soil which can be used utilised elsewhere on the patch. Once that’s done, the soil will need to be dug over carefully.
Precisely making sure you eradicate any roots left from the pestering perennial weeds such as marestail (which looks like a one-stem fir tree), couch grass, Japanese knotweed (tall, green and pink stems looking slightly like rhubarb as it emerges), bindweed (bright green, shield-shaped leaves and white trumpet flowers) or ground elder (like a large flat-leaved parsley with mini, white, cow-parsley-like flowers).
To really control these weeds depending on if you want to go down the non-organic or organic route. You can try spray them off with glyphosate (found in systemic weed killers) available at any garden centre. Mix this to the recommended strength on the back of the pack and on a very still day, very carefully spray off your patch, laying weighted-down plastic sheeting around it to prevent any drift onto the surrounding grass or flower beds. Leave it for two weeks or until any green has browned, then dig over the ground.