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> Although I'm not sure where I might be able to customize ligatures such as my own "At" and plural "s", it's something to consider.
The use of ligatures in fonts is an interesting topic. The way intended for the future is to use a feature known as glyph substitution. However, ligatures can also be used by direct mapping: this can lead to non-standard mappings yet can be useful for some purposes, such as producing hardcopy printout locally.
The Font Creator 5 program does not support glyph substitution features. Also, even if a font does support glyph substitution it is only of use if the software using it can also support glyph substitution. However, as time proceeds maybe a later version of Font Creator will support glyph substitution features.
However, at present I would encourage you to produce such ligatures as you choose and make each of them accessible to a program such as Wordpad using a mapping in the Private Use Area. That area is from U+E000 to U+F8FF, though it is probably best to place your ligatures in the region U+E000 to U+EFFF.
In addition, at some future time when glyph substitution features are perhaps available and an updated font is perhaps being made, those same glyphs would be available to be referenced in a glyph substitution table. This would be easier than attemting to draw the ligatures at that time as, in my opinion, it is best, for handwriting fonts, to try to do all of the artwork at one session and all of the glyph production at one time if possible: this is so that any choices made perhaps almost subconsciously are done together within the same train of thought.
Also, it is possible to make glyphs which are accessed using glyph substitution also to be accessible using a Private Use Area mapping, so that direct access from an application program is still possible.
> and plural "s", it's something to consider.
Now that is an interesting thought.
When I have seen examples about glyph substitution they have always had sequences of glyphs which end with a character which "uses ink". If a sequence such as small e small s space can be used for glyph substitution then an "es for the end of a word within a sentence" ligature could be used.
Example of fonts with ligatures within the Private Use Area are my Quest text font, my Chronicle Text font and my Style font. Quest text is rather large so maybe Style and Chronicle Text are easiest to study. In each there is a glyph for a ligature of small c small t at U+E707. The designs of the glyphs are very different within the two fonts. Please note that Chronicle Text is one of my older fonts which started off in the Softy program. The Style font has been produced entirely using Font Creator 5.
Each of those fonts has a thread in this Gallery forum.
The fonts are all available from the following web page.