Converting your land plane to floats is not as hard as it might sound initially. If you have a set of commercial floats (like GB's or Sullivans), then you simply install them as per the manufacturer's instructions (or did you throw that out already?).
If you need to choose commercial floats, or are building your own, there are a few design considerations to keep in mind. These rules of thumb are summarized here and pictorially shown in the figure that follows:
Side note: You can calculate these dimensions from basic physical principles! One cubic foot of water weighs 60 lbs. If your floats weigh 1 lb/cubic foot (typical for foam floats), then a 60 lb plane on those floats will only need to displace 1 cubic foot of water. Factor in a factor of say, 2-3 for safety (after all, you don't want your floats to be totally submerged), and this is how much float volume you need! Adjust the width as needed to achieve this volume.
For Styrofoam floats, be sure to groove the top of each float to accept the plywood stiff backing. Epoxy this ``backbone'' in place, then drill out 1/4'' holes and insert 1/4'' dowels into each hole. Without the dowels, the first really hard landing will tear the backbone from the Styrofoam.
Finishing the floats is up to you. Styrofoam floats are often plated in balsa and painted or monokoted (use a low-temp covering to avoid melting the foam). You can attach them to your existing landing gear via landing gear brackets and sheet metal screws. Tricycle-gear planes will need replacement nose gear (see the diagram below), and taildraggers will need a second (aft) mounting plate on the fuselage. You will want to attach a cross-brace across the landing gear to keep the floats from ``spreading'' out under heavy load or choppy water. Some of these hints are depicted below, adopted from an article by Chuck Cunningham in last year's RCM magazine.
There are as many ideas concerning how to build floats as there are different kinds of airplanes! So if you think of an interesting idea, try it out! Next month, we'll explore the inevitable subject of how to takeoff and land on water, with crosswinds. We'll also discuss what kinds of planes make excellent conversions to water... [Hint: the plane shown at the beginning of this article is an electric... wicked grin]