Dig out strips of turf wide enough to accomodate the timbers. Pressure-treated, softwood sleepers are an economical alternative to rot-resistant hardwoods like oak. You could also consider buying reclaimed hardwood.
Lay the timbers out and check that they are level using a spirit level, or a plank of wood supporting a shorter spirit level. Check the levels diagonally between timbers, as well as along their length.
Using a rubber mallet, gently tap the wood so that is butts up against the adjacent piece; it should stand perfectly level and upright according to the readings on your spirit level. Remove soil as necessary.
Drill through the end timbers into the adjacent pieces at both the top and bottom to accommodate a couple of long, heavy-duty coach screws. Screw firmly into position, securing the base ready for the next level to be built.
Arrange the next set of timbers, making sure that these overlap the joints below to give the structure added strength. Check with a spirit level before screwing in the final set of fixings, as for step 5.
For the extra drainage required by plants, such as Mediterranean herbs and alpines, part fill the base with construction rubble or chippings. Then add sieved topsoil that is guaranteed free from perennial weeds.