Do you have problem with this words:
I overcome the problem this way:
S and non are the letters I can remember easily.
Syn is hard for me!
I have to convert Syn to Sin, and the sound of sin help me to remember the word.
nym is also hard for me!
I have to convert nym to nim, and the sound of nim help me to remember the word.
As "non" is the one that connect and overlap, hence
Sin + "non" + nim = Sinonim, I then convert the "i"s back to "y"s
It is a round about way, yet it works for me!
You may find your own way!
Just keep trying!