Tri-fold futons are incredibly useful pieces of furniture. Not only can the be used for a fairly comfortable bed (a lot depends on the mattress in the case of comfort), they also can be reconfigured to make a small couch for sitting on. The one thing about tri-fold futons is that they have a lot of moving parts and you have to do most the assembly yourself.
Look over the packaging slip and the contents of your futon. You should have six framing boards, enough slates to cover the entire frame and the futon hardware. If you get a bed and there is no hardware, or pieces are missing, call the seller or the manufacturer.
Lay out the pieces of wood. It's good to have an idea of what the frame will look like before you start to permanently affix slats to the frame. The frame boards for most futons are marked with small dots where the slat boards are supposed to affix with the screws.
Affix the slats on to the frame boards with the cordless drill. The screws that hold the slats to the frame are generally Phillips head screws, but some use Allen head screws also.
Connect the frame points with the included Allen wrench and screws. Typically the frame of a tri-fold futon is divided in three sections, with the middle section frame boards being either farther apart or closer together than the two other sections. This allows the futon to bend. To connect the frame points, line up the outer frame hole with the inner frame hole on the middle frame and affix the screws.
Check to make sure that the completely assembled bed folds as specified in the directions.
When attaching the frame points, never use a cordless drill. Too much torque at this point can cause the screws to strip out the holes, and the frame will never attach properly once the holes are stripped out and widened.