1. Cut clean edges and define the boundaries
Before you plant or buy anything, cut the edges. This is the single highest-impact hour in the whole list and it costs nothing but effort, because the eye reads crisp lines as care and blurred ones as neglect regardless of what is growing.
Work out where the beds should stop and the path or lawn should start. Lay a hose or a length of rope on the ground to trial the line and look at it from the pavement and from an upstairs window. Curves should be gentle and few — a small front garden with four separate wiggly beds looks fussy, and one generous bed with a clean sweep looks considered.
Cut along the line with a half-moon edging iron or a sharp spade, pushing straight down about 4 inches and angling the cut slightly away from the bed. Lift out the strip of turf and knock the soil off it into the bed. Sweep the path.
Then deal with the boundary. A low picket fence, a rope-and-post edge or even a run of reclaimed bricks set at an angle costs $40 to $80 and stops the lawn creeping, stops dogs cutting the corner, and gives the garden a visual frame. Check whether the boundary is yours before altering it, and in the UK check for any covenant on front boundary heights.
Finish by trimming back anything overhanging the path at head height. A visitor should walk to your door without ducking.