The main trick to building it properly is getting the big ring just right. Glue matching top and bottom halves together. When they're ready, work on a large flat surface so you can work all three pieces at the same time.
Lay down either a large piece of art board or tape a bunch of sheets of printer paper together. You want to cover the area the ring takes up so you're looking at something on the order of 24" square. Draw a crosshairs on your work surface large enough to span the diameter of the ring - 16 inches. Use a string, pencil and thumbtack to make an assortment of concentric circles, each about 1/4" smaller than the last, until you have about two to inches worth of circles from 15" to 17" diameter. Last, use a protractor to draw diagonal lines out from the center at 120 and 240 degrees.
Now lay out your three ring sections. Have some .010 x .060 Evergreen or Plastruct strip handy. Between sanding off kit part material and shimming with the strip plastic, keep working the ring until it perfectly aligns with the circles and 0/120/240 lines. As you find the ideal position for each piece, masking tape it down to your work surface.
When you've got everything perfect, dab glue on the joints to lock them in. Let sit for at least a full day, more if possible.