Removing eisiting surface (lawn or soil) to a depth of 100mm and level the surface. it may help to hire a whacker plate/compactor to complete this task. In cases where the exisitng ground does not drain well, it may be worth digging out further to aid damage.
Batten out the area to be covered with pressure-treated (tanalised) timber to create a frame - the purpose of the frame is two-fold as not only does it help to contain the aggregate but it also provides a secure frame onto which the grass can be fixed using the galvanised screws.
At this stage, you can either simply weedkill the area or, alternatively, install a weed-suppressing membrane to the sides of the frame or alternatively, the weight of the aggrgate and sand layers will hold the membrane in place.
Apply a good quality fine aggregate (25mm-75mm) and levek and compact again. Apply a further layer of sharp sand (25mm) and level and compact again.
Once the aggregate/sand sub-base is level with top of the battened-out frame, lay the grass in place and trim to fit, where necessary, using a sharp knife and cutting from underneath. Trim back gradually until the required size is achieved.
Screw the grass to the timber frame using galvanised screws.
You can apply a top dressing of klin-dried sand which is readily available from builders merchants. Whilst optional, the sand helps to hold the grass in position as well as maintaining the pile upright. If using, simply apply the sand evenly over the grass and working against the pile, using a stiff brush.