There are a number of ways of making a cutaway. The easiest, and, perhaps, oldest, type to make is to have a discontinuity where the body meets the neck, like this:
That is, there is a vertical corner on the treble side of the neck/fingerboard, where the hand bumps up against the corner. This is accomplished by making the side proud of the edge of the fingerboard and was done to accomodate the dovetail joint that was used. There's nothing "special" about the construction. Regardless of neck juncture, a short piece of side is used between the neck block/interior of the Spanish heel/foot and the heel. The cutaway abuts the short side piece at a corner. If the heel shape is heavily (traditionally) contoured, much of the short piece of side is visible.
Many more recent designs have the cutaway side flush with the fingerboard at the neck/body juncture, like this, but the vertical corner of the cutaway still exists:
If the heel is the same width as the fingerboard for the entire depth of the heel, none of the short piece of side is visible under the heel, as shown below and is often more forgiving to make than some other options.
https://www.guitarfromspain.com/6232...nco-guitar.jpg
A more difficult, still, approach is to have the cutaway match the contour of the treble side of the fingerboard as well as the (traditional) contour of the heel. It requires the end block or Spanish foot to be undercut the thickness of the side so that the exterior surface of the side is flush with the fingerboard and heel. The side must then be bent across its width - to match the contour of the heel - as well as length. One example is shown here:
http://charlestauber.com/luthier/Home.html.
More "modern" cuttaways involve having a cutaway that does not extend the full depth of the side, such as shown below. As far as the interior block/Spanish foot is concerned, it is built the same way as a non-cutaway.
https://static1.squarespace.com/stat...1493939946318/
Probably, the starting point is to decide what sort of cutaway design you want to make. The details of how to accomplish follow from that. Unless you have a specific reason for wanting to use a Spanish heel/foot, I'd suggest using a separate-neck construction.