This jig is designed to be used with a router equipped with a 5/8" O. D. Porter Cable style guide bushing and a 1/2" Diameter mortising bit.
 

To make the jig, first obtain a piece of hardwood lumber (preferably Maple or Birch) approximately four inches wide and four feet long.

Layout the board as shown in Fig. 1.  Begin by cutting two pieces (A) and (B) about 3-1/2" longer than the diameter of your router base plate, and about half as wide as the base plate.
 

Fig. 1
 

Joint and dimension the remaining board, from which pieces C and D will be cut to a width equal to the width of your hinge leaf plus the difference between the diameter of your router bit and the outside diameter of your guide bushing. E.g. (5/8" - 1/2" = 1/8")

Note: that the width of this board is critical to your success in using this jig.  If the board is dimensioned to narrow the fit will be tight, and if the board is to wide the fit will be loose . 
 

Cut one piece (C) about 3-1/2" long from the remaining board.

Glue the three pieces (A) (B) and (C) to form a U shaped assembly as shown in Fig. 2.  After the glue is dry you can plane the assembly on both sides to clean up any mismatch in gluing.
 

Fig. 2