I have added a section to the tutorial on Adding OpenType Features about adding standard ligatures.
Many fonts will have glyph substitutions to replace f+i with the fi ligature found in Alphabetic Presentation forms, and f+f with the ff ligature, f+f+i with the ffi ligature, etc.
Because OpenType features do not require code-points, but work with glyph names, you don’t need to map your non-standard ligatures to any code-point, but if you wish to be able to insert them in applications that do not support OpenType features, you will need to find an unused code-point in the Private Use Area or elsewhere.
The lookup table for your font would look something like this:
sub g O → goto
sub s I n → sin
sub c O s → cos
etc.