FAQ2:  How do I rig my Floats?

Floats don't only keep the airplane from sinking in the water when you're sitting still, but they act as water skis when the plane is landing and taking off, and they must be also compatible with the plane's flight.

You have read FAQ1 about the right size for the floats.  
Now you  figure out where they go.  

Short answer: On the bottom, Fat end in front.  

Longer answer:
The first thing you do is to locate the step.  It's the notch in the bottom of the float.  When the plane is going fast, it planes on the step.

Step:  The commonly accepted guideline is to locate the step under about 40% of the effective wing chord. Fig1.  Don't panic, it's not that hard.  If you have non-tapered wings,  the wing chord is the distance between the leading edge and the trailing edge.  So if you have a .40 sized trainier and the wing is 10" wide, the step goes under a point about  4" back from the leading edge.  If you miss it by a half-inch it's not the end of the world.  Pretty much it is known that it will work at the 40% point.  If you have a more advanced plane with tapered wings, put the step about an inch behind the balance point.  If you have engineering training, compute the mean aerodynamic chord and put the step under the 40% point.    

Spacing: The common guideline is 25% of the wingspan.  That's kinda limiting and nobody expects you to scrounge around for narrower landing gear if you have a clipped-wing cub.  
Pragmatic answer:  Use the landing gear spacing that the plane came with.  Figure 3.

Parallel:  Make sure the floats are parallel with each other and with the centerline of the plane.  Some flyers have advocated a little toe-in, or having the noses of the floats closer together.  I can't say it WON'T work, but I have  had success with parallel floats.  

Incidence:  When the plane is scooting along on top of the water, the wing needs to get enough of a bite of air to lift the plane.  So we set the floats in the takeoff attitude ( usually with the tops level) and tweak the wing up with 2-4 degrees of incidence (shown as 0-5 degrees in Fig 1.).  If you have a flat-bottom airfoil the actual incidence is from the true leading edge to the trailing edge. So having the bottom of the wing parallel to the top of the float  is frequently just about right because the actual wing incidence is different than the bottom of the wing.  If your plane has a wing that is curved on the bottom, make sure that the leading edge sits higher  then the trailing edge with the tops of the floats level.  3 degrees is really close to 1/2" incidence on a 10" wing.  It might be easier to rig the plane if you remove the wing and measure from the wing saddle.   Another method for setting the incidence is to set the horizontal stabilizer parallel to the tops of the floats.  Sometimes that's easier because many planes have a flat-plate stab and it's easy to set a torpedo level on that to rig the incidence.  
IF YOU DON"T HAVE ENOUGH INCIDENCE: The plane will likely refuse to take off unless the water is choppy.  Many call this "suction".  What it really means is that your float installation sucks. Also, some goober will tell you that you need a bigger engine.  You don't, unless your plane won't fly except at Wide-Open-Throttle.  
IF YOU HAVE TOO MUCH INCIDENCE: The plane will become unstable during the takeoff run because it rides on the noses of the floats at high speed instead of riding on the step.   If this happens, hold a little back-stick on the takeoff run.  Porpoising on the takeoff run is also an indicator of too much incidence, and also resolved with a touch of up elevator until you fix the floats.

Fig1fig3

Good news: there is about a 6 degree window of opportunity on the incidence where the plane will work anyway.  

Prop Clearance:  Other texts will tell you it is necessary for the prop to  be some distance behind the nose of the floats to keep the plane from nosing over.  Maybe that helps, but it SURE keeps the prop from chewing up the dock when you are taxiing in.  If you sized the floats correctly this will take care of itself, but 30% of the prop diameter is a good value to watch for.  Likewise the height of the bottom of the prop above the plane of the tops of the floats should be about 20% of the prop diameter.  Labeled 2" in Fig 1, but an electric with an 8" prop needs less clearance than a quarter-scale with a 20" prop.  This is primarily to keep the prop put of the spray that squirts out of the sides of the floats.