strip out the plants and any weeds, including their roots, dig the soil over, level it. If you can, leave it for a week or so, then rake it over, removing anything that might be growing, walk all over it to get out soft spots, relevel then lay turf, trying to ensure the new turf ends up the same level as the rest of the lawn, and keep well watered till end of autumn. But its very difficult to get new turf to sit at the same level as existing turf... it usually either ends up lower or a bit higher.

Thank you , what would be the best month to do it

october's the best month to lay turf, but its often too wet to do it - I'd make a start on the preparation in the latter half of September