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Since you’re going to be sending sawdust everywhere, work outside where you have plenty of space and ventilation. Set up your sawhorses in a spacious area of your garden or yard. Put on a dust mask and protective eyewear. Lay your 1 in × 4 in (2.5 cm × 10.2 cm) fence boards flat on your sawhorses. Use a measuring tape to measure the board into 6 different pieces of equal lengths. Use a straight edge or angle square to mark a straight line where you want to cut with a carpentry pencil.  4 of these pieces will make the longer sides of your box, while 2 of them will serve as the base. The size of your sides will determine how long the length of your box will be. A length of 3 feet (0.91 m) is good for a medium-sized box that will fit 3-5 smaller plants. You can use pine, spruce, or fir, but cedar is generally more durable and is better at handling intense weather over long periods of time. Use pressure-treated wood if you don’t plan on eating anything that comes out of your planter box, since it will hold up better over time. You don’t want the chemicals used to pressure-treat wood seeping into any fruits, vegetables, or spices that you’re growing, so don’t use pressure-treated wood if you’re growing any vegetables, fruits, or spices. Press the edge of your circular saw’s base plate up against the line you want to cut and pull the trigger to fire the saw. Wait 2-3 seconds until your saw is spinning at maximum speed and move the saw forward to start your cut. Use the guide line on the front of the base plate to guide your cutting line. Repeat this process for each cut until you have 6 pieces of equal length.  Set 2 of the lengths aside. These will be the bed of your planter box and will come in handy later. You can use a miter saw instead of a circular saw if you prefer. You can use the first length as a straight edge to cut your other pieces so that they’re uniform. Don’t push forward too hard as you cut. Let the traction from the saw blade pull you through the cut. Use a measuring tape to measure out 4 lengths of equal size on 2 in × 2 in (5.1 cm × 5.1 cm) cedar posts. Use a straight edge or angle square and a carpentry pencil to indicate the location of each cut.  The longer you make your pieces, the higher off of the ground your box will sit. 14 in (36 cm) pieces will make your box sit relatively low to the ground while still keeping it elevated off of the floor. You can also mark off one cut and then use a single piece as a straight edge for each additional portion. Rub a file against the edges where you cut to remove any splinters and even out any rough edges. Place each section of your 2 in × 2 in (5.1 cm × 5.1 cm) board flat on your sawhorse and put the edge of your circular saw’s base plate against the line that you marked. Pull the trigger on the saw and carefully run it through your guiding line. Repeat this process for each section until you have 4 separate pieces. These 4 pieces will serve as the individual legs for your planter box. You will install them inside your box to keep the frame together. You’re going to join your 2 sides together with 4 smaller pieces. You’re going to stack 2 of these smaller pieces on top of one another for each side, so you need 4 pieces total. Measure and mark your lengths on the fence board with a measuring tape and straight edge and cut them the same way you cut your other pieces.  These 4 pieces will be used to form the sides on the 2 shorter ends of your box. 12 in (30 cm) is a good length for your fence board pieces if you want a medium-sized planter box. The ends will determine the horizontal depth of your planter box, so you can use larger pieces to make a bigger box, or smaller pieces to make a shallow planter.
Set up your work station outside and wear protective gear. Set up fence boards on your sawhorses and measure 6 pieces. Cut your 6 lengths with a circular saw. Measure 4 pieces of cedar to make your legs. Cut your cedar boards into 4 pieces of equal length. Measure and cut 4 pieces of fence board for the shorter sides of your box.