Ground preparation to produce a clean seed bed has two components: site clearance and soil cultivation. The time needed to complete this can vary from a few weeks to years depending on the condition of the site and the type of weeds present.
In open ground control of 'weeds' can be achieved by repeated applications of a herbicide (e.g. glyphosate) over the growing season and/or repeated cultivation to exhaust weeds (fallowing).
Recently cultivated land with annual weeds can be cleared for sowing in a few weeks.
Neglected plots which contain perennial weeds with extensive root systems (eg couch grass) may need more than one treatment and take more than one season to complete. It does not pay to rush this process as it is very difficult to deal with residual perennial weeds after sowing.
Unwanted vegetation can be cut back and removed from site or chopped up to be incorporated during cultivation.
Cultivate the soil to sufficient depth to bury remaining trash and to alleviate compaction, then rake or harrow and roll to produce a fairly fine, firm surface.
The finished seedbed should be firm enough to walk on without leaving impressions. As the area will be mown at a later stage the finished surface should be free of obstructions such as large stones or bricks, and free of deep ruts or ridges.
The "stale seedbed technique" can work well for annual weeds whose seeds will remain in the soil after clearance. This method involves preparing a seedbed then delaying sowing to allow a flush of weed seed germination from the surface layers. This flush of weeds is then killed, by spraying or shallow cultivation, before sowing your seed mixture onto the cleaned "stale" seedbed - the surface of which now has a reduced weed seed burden.
Soil cultivation around established trees and hedges can be very damaging to tree and shrub roots - only surface treatments are safe here.