Recently I worked on a project that required installing a pair of “Card Table Hinges”. These hinges sit flush with the top and are mortised into both halves. To create this mortise, all you need is a router and a shop-made template.
The template is simply a piece of 1⁄2" plywood with an opening sized to match the hinge. You can use one of the hinges to trace the outline onto the blank. I used a Forstner bit to drill a hole at each end and removed the waste with a jig saw. Then sand the edges smooth with a sanding drum or by hand sanding.
To use the template, clamp the two halves of the top together. Then attach the template to the pieces with double-sided tape. As shown in the detail, a dado clean-out bit makes quick work of routing the shallow recess for the hinge leaves. To create the deeper mortise for the hinge knuckle, first mark the edges of the mortise. Then, adjust the bit for a deeper cut and rough out the bulk of the waste, stopping just shy of the layout lines. Finally, you can square up the edges of the mortise with a chisel.