You can establish a crisp edge to a garden to show a distinction between grass and flowerbed with a shovel, spade or power edger/trencher. You need to wait until your soil is reasonably dry. The process requires digging a shallow trench to cut through grass roots. For long garden beds, the easiest and fastest method is to use a powered edger/trencher.
Open the door of the edger/trencher while it is unplugged and clean out any dirt with your fingers or a stick. Close and lock the door.
Set the blade depth to the deepest available setting and select the trench setting. Plug in the edger.
Place the blade portion of the edger at the location where you want the break between grass and the garden to start. Hold the handle of the edger down to allow the blade to be slightly above ground. Squeeze the trigger to start the blade spinning and slowly lower it into the ground.
Push the edger forward at a slow and steady pace. Make several cuts in this fashion, so you can finish the length of the garden edge or make multiple cuts in small sections. With each subsequent do-over, move the edger slightly further into the garden or grass to create trench about 6 inches wide. Cut a three-foot stretch if you prefer and work that area repeatedly to gain the trench separation between the grass and the garden.
Clean collected dirt from the blade compartment periodically if the soil is moist. Unplug the edger before you open the door.