Decide for the dimensions. A standard twin mattress size which can be used is 39″ X 75″, but pick your mattress first, then your pallets. Pallets also have various sizes, but 48″X40″ is fairly standard. You will need 5 large (4″) casters (2 which are immovable and three that have locks), some outdoor polyurethane and a brush, and sanding discs in 60, 180, and 220 grits, an orbital sander.
At first sand those pallets down which really need to be smoothed out. Start with the lowest grit (60) and then ascend up. Wearing safety gears for eye protection and a mask. Pick a drill bit, and lots of bolts and nuts, and a pile of washers. Three bolts are needed for attaching the two pallets together, and bolts for attaching the casters to the pallets, and enough washers and nuts to keep them all intact.
Cut your pallets to size first. Attach the sides with the wood frame. As per discussed measures the base would be 48″ deep and 80″ long. Cut the 48″ side down to about 40″ to accommodate your twin size mattress. For a standard mattress the final bed base would be 40″X80″. Drill three holes with your 3/4″ drill bit to attach the two pallets together.
Carefully measure and mark three holes on each pallet, one close to the front, one close to the back, and one in the center. Drill the holes with your 3/4″ drill bit. Join them with the help of nuts and bolts and tighten it with wrench properly. Place the casters on each corner attaching them with the bolts and then tightening them with washer and nuts on all four corners. This completes the base of your bed. The surface can be protected by several protective layers of polyurethane. You can have the arms attached to the bed and decorate it as per your garden decor.
So enjoy your daybed!