To make a definitive border around your garden bed to separate it from your lawn there are a lot of options to choose from. From concrete and brick, to pavers and gravel, possibly the easiest and cheapest method is to install plastic border edging. With a plastic edging that goes into the ground you can keep wandering plants from spreading roots from one bed to the other and make the task of mowing around your beds easier.
Measure the full length of your garden bed, making note of where the corners of the bed are, since this is where you may need to cut an edging piece. Purchase as many pieces of edging as needed to run this length.
Dig a narrow trench around the edge of your garden where you measured which is as deep as your edging pieces. Typically you will only need to dig three to four inches deep.
Start on one end of the bed and begin setting in one border piece at a time, or connect three to four of them together before setting it into the trench.
Align the pieces so they run as vertical as possible without tipping over and only a half inch sticks up above the ground level. Use a rubber mallet or push down with your hands as needed to help set each piece into place to secure it.
Continue to add more pieces of edging until you reach a corner. If the pieces line up well, you may have a piece end right where the corner is, making it easy to start along the other side. If your piece doesn't end where at the corner, use a hacksaw or jigsaw to cut the piece in half and start the next side with the unused half of edging.
Finish setting in all of your edging pieces before filling in the trench around your edging by adding back in the removed soil from step two. Pack the soil tightly around the plastic edging to hold it in place.