I’m trying to create a font handwriting. For some letters I would like to create ligatures, for example “ss” or “tt” I tried to follow the tutorials but I can not. I create a new glyph with tt but if I try to change the letters the original t changes too, while I would like to insert a combination of two “t” combined with each other different from the original one. I hope I was clear. If you can explain to me how you would do with a child, thank you!
- Create a glyph called
t_t(the underscore is important). - In the glyph, decompose components (Glyph > Decompose Components, Cmd-Shift-D).
- Now you have outlines, which you can edit and adjust.
- In File > Font Info > Features, click the buttons Update and Compile.
Thank you for the answers, i try but and now i’m able to change letters detail without change the original. but when I finish the steps you indicated to me and i type words with tt remain the same as before 
You have to activate the ligatures. It is described in this tutorial:
https://glyphsapp.com/tutorials/ligatures
Ok thanks a lot! the last thing, if i want substitute some of last letter of word how can i do?
That is a little more difficult. There is a tutorial called Positional Alternates that describes steps.