You answered your question.  :)  Indeed you need to plan carefully which ligatures shall have priority.
Don't forget the space between 'a' and 'm', or 'c' and 'a' -- 'am' would be considered as one glyph named 'am'.

One very common mistake is to mix up the priority of overlapping ligatures, e.g.

sub f f i by f_f_i;
sub f f by f_f;
sub f i by f_i;

If that ffi ligature is not first, it will get preceded by the others and never appear.

Also, beware of the output of other alternate features (e.g. 'salt') affecting the input of ligatures. You might want an alternate f to be included in your ligature substitution -- or you might not. If you have contextual substitutions ('calt', 'clig'), then you'll have to worry about other alternates and ligatures affecting the context in those features, too.