This is a simple, half-day project that begins with a scrap oak pallet, scrap 4 x 4s, scrap 2 x 4s and scrap 1 x 4s. The only other materials required are 8 - 3/8" x 4" bolts with washers and nuts; 8 - 2 1/2" and 8 - 2" Deck screws; and a quart of spar varnish (if you want a fully finished look).
You'll need the following tools: a good hammer; a nail punch; a circular saw (you can use a hand saw if you're really ambitious); an orbital or belt sander - I prefer the latter for this project - or you can hand sand, but I wouldn't recommend that unless you have lots of time; a cordless variable speed drill (if you want it to double as a screw driver) or use a standard 1/2" drill and a sheetrock screw gun or hand screwdriver; 3/8" and 1/8" wood drill bits; a counter sink bit; a socket and open end wrench set (SAE, unless you are using metric bolts); and a pair of saw horses or other suitable working surface.
Step 1: Mark the Cuts
Place the pallet on the saw horses and use a carpenters square to make a mark on the pallet cross-slat even with the inside of each of the center 2" x 4" stringers on the pallet at each end. Then, using either a straight 2" x 4" board or a 4' Level, mark a line on the slats using the end points already marked on the end slats above each stringer. You will repeat this process on the flip side once you have completed the first cuts.
Step 2: Separating the Pallet - First Step
Using a circular saw with a rip blade make a cut across the slats on each of the marked lines. Go slow because sometimes the stringers are warped and could bind the blade, but if you are careful, the most that will happen is that you'll take a thin slice out of the stringer, which can be smoothed with a belt sander later.
Step 3: Separating the Pallet - Second Step
Take the pallet off the saw horses and put a 2" x 4" x 4' support board on each saw horse and clam in the center to prevent movement (I used bar clamps - they have been removed for the picture), then place the pallet back on the sawhorses with the reverse (uncut) side up. Using the circular saw, make the cuts on this side as you did on the previous step. This will separate the pallet into two bench-sized sections. Remove the pallet pieces and support boards from the sawhorse.
Step 4: Smoothing the Rough Boards
You will need a belt or orbital sander to make this step easier. Before sanding, however, use the nail set and hammer to drive all nails below the surface of the wood so you do not tear up your sanding belt or paper. Once that is done, proceed to sand, using a course grit first, then a medium grit to finish (this is a garden bench, so using a fine grit to finish is kind of gilding the lily).
Step 5: Preparing the Legs
First, measure and cut four 4" x 4" boards (posts) to the same length. Since I wanted a bench that could be used to sit on or as a plant stand, I cut mine to 18 1/2" in length. Next, line the legs up so they are even on the ends and clamp them together with 2 bar clamps. Next measure and mark the exact center of the 4" x 4" length on each end of the stack (in my case that was 9 1/4"). Next measure and mark a point 1 3/4" from the center on each side on each end, then using a straight edge or 2' wood working square mark lines from each of the outside marks on each end. This marks the inlet channel for the 2" x 4" support beams on the legs.
Use a carpenters square to make sure your saw blade is exactly 90 degrees to the base plate of the saw, then set the saw depth at 1 3/4" and cut along the drawn lines (the kerf should just touch the line from the inside) across the entire stack. Then make several cuts between the lines so you leave only thin wafers between the lines.
Take your hammer and knock out the wafers, then buzz with your saw (keeping the saw depth at 1 3/4") by moving it side to side in the channel until the bottom is smooth and even over the entire stack. If you are uncomfortable using your circular saw to do this, you can use a chainsaw, being careful not to dig in, or a wood chisel. If you do either of the latter, mark the depth with a line prior to buzzing or chiseling.
Step 6: Installing the Legs
To be sure your bench sits evenly, clamp each leg (one at a time) snugly, but so the leg can be adjusted, and use a carpenters square and/or torpedo level to check the alignment for vertical. Then tighten clamps and drill two 3/8" holes diagonal from one another through the outer stringer and the leg. Place a washer on each bolt and then insert the bolts into the holes, with the bolt head to the outside. Place a washer and nut on the thread end of each bolt, then tighten using wrenches. Holding the head with a box-end wrench and using a socket wrench to tighten the nuts makes the job go faster and ensures a tight mating of the parts. Repeat the process for the remaining three legs (but do one side at a time including installing the cross braces - next step). This actually goes pretty fast if you install one set of legs and its cross brace at a time, and it makes setting the brace so much easier.
Step 7: Setting the Cross Braces
Once the legs for one side are installed set your 2" x 4" brace in the inlet channel - if you made your cuts correctly, it should take a tap or two with a hammer to seat them. Clamp the brace in place. Drill two 1/8" pilot holes in each end of the brace at diagonals (see picture with screws ready to be snugged). Start a 2 1/2" Deck Screw in each hole, then drive in with a screwdriver (I like to use my variable speed dry wall driver because it gives me greater control).
Repeat the previous step and this one for the other side of the bench.
Step 8: Final Step - Installing the End Braces.
This last step makes sure that your bench will be solid and sturdy. Measure the distance from the outside of one leg to the outside of the other on the narrow ends of the bench and mark two 1" x 4" boards to fit across them. Cut the boards. Clamp the boards in place to match the 2" x 4" inlet braces, then drill a 1/8" pilot hole in each end of the boards. The holes should be drilled diagonally as was done with the inlet braces. Set the braces using 2" Deck Screws. Set the bench right side up on the floor, varnish it if you are so inclined, and you're done... you have a nice bench to set your plants on or throw a couple of outdoor cushions on it for a garden love seat!