Welder Guy is right, downhill. These are probably the easiest welds, too.
Make sure the metal is clean and free of rust. On butt joints, bevel the edges to ensure penetration. If you tack up without beveling, use a grinder, up on edge, to groove the joint. For outside corners, butt the inside edges of the two pieces together, sort of forming a bevel. Makes a very nice weld. You can even leave a slight gap for max penetration. Inside corners, just go for it. Keep the voltage/wire speed relatively high. Tune up the wire speed and voltage. It's simple, if you don't know how, ask. Use a 1/4 to 3/8 wire stick out from the contact tube to the metal (technically it's to the top of the arc, but that's nit pickin)This is important, on downhill, always keep the arc (squirt the wire) on the leading edge of the puddle. Definately take a few minutes to practice and set the machine up. Try breaking some of your practice welds so your confident it's working. Run as high of a voltage setting as you can handle the amount of weld build up. Manufacturers recommended settings are typically a little cold, and my MM250 runs 2 to 3 volts lower than indicated by the dial. What your attempting to do works, and actually, it's a lot of fun.