A recent commission required four locker doors to be cut off a racing yacht main cabin back seats. I wanted to have a clean design and use an already existing shape. A Tupperware cover, a serving plate, a cutting board… can be used as a initial template. Doors dimensions will be larger than the template by about the width of the collar. Actually, the perimeter of the opening will be larger than the template by X, all around the template.
X = Outside diameter of the collar – ½(outside dia. of the collar + dia of the bit )
              Or
X =Æ Bit + ½ (Æ collar - Æ bit)

I'll describe the use of bushings and templet guides. The process is quite identical with flush trim plunge bits and ball bearing guides.  

WINDOW - "HOLE PIECE" TEMPLATE
Draw around the template
Final opening will be larger than the template by "X"
Trace a cut-out contour 1/8" INSIDE the final opening
Waste is on the inside
Cut-out contour with a jig saw - Drill a starting hole to insert the blade. 
Use the Larger Collar and rout-out the inside of the window

Use the template for multiple windows:
Position the window template on final piece to cut out
Secure with clamps and/or double face tape
Cut-out inside with jig saw
Clean the edges with the router equipped with same large collar and a straight or spiral bit

DOOR - "FILL PIECE" FROM TEMPLATE : NO JIG
Draw around the template
Final piece will be larger than the template by "X"
Trace a cut-out contour 1/8" outside the final piece
Waste is on the outside
Cut-out contour with a jig saw or table saw
Attach the template on the piece with double-face tape, screws, hot melt glue…
Use the Smaller Collar
Clean the edges with the router

DOOR - "FILL PIECE" TEMPLATE
Draw around the template
Final piece will be larger than the template by "X"
Trace a cut-out contour 1/8" outside the final piece
Waste is on the outside
Cut-out contour with a jig saw or table saw
Use the Smaller Collar
Keep the Fill Piece as a Guided-bit pattern for router table or a flush-trim bit.