Metal futon frames are among the cheaper futon models available. These are the (usually) black frames with tube-like parts, often found in larger stores. They also usually come unassembled. The simplest option is to have someone assemble it for you, if the store offers that service, but this is not always free. This leaves the assembly up to you.
When putting together a metal futon frame, get someone to help you, even if you don't think you'll need the help. If you've seen a futon salesperson assembling a frame by himself, you may think it simple for you to do as well. However, the salesperson has probably assembled many more of these than you have (and thus has it memorized), and the pieces may be too heavy for you to hold up or move. Even if you are strong enough, the size may make assembly awkward if you are the only person holding together several different parts of an unfamiliar frame. As you get further into the assembly process and possibly begin to sweat, the smooth metal of the frame may become slippery.
Futons aren't always used as a dual couch/bed; you might decide to use yours only as a bed. Some metal futon frames have a small metal bracket that you place between the two main panels when the frame is flat. The two legs of the three-sided bracket fit in holes in the main panels and help steady the frame, in case someone accidentally hits against the frame hard enough to lift the panel and start the folding process. Keep in mind the bracket is small, so if the frame's folding mechanism won't allow you to keep the frame flat, get it checked out as soon as possible, rather than relying on the bracket.
Properly set things up before assembly. Once you remove the parts from the box, you'll find yourself surrounded by bits and pieces; don't place them on varying surfaces all over the room, where they can fall and you step on them. Despite how common metal frames are, you may not be able to simply walk into a store and pick up a missing bolt, so ensure you have all the parts and tools you need. The parts may also be dirty or covered with oil; keep paper towels nearby. Wear gloves to protect against cuts, in case a metal edge is ragged or sharp.