The letter ç (as in "français") is replaced by an s, which is doubled if the ç is between two vowels (if it is preceded by an e, it will be prounced /e/).
The letter c (as in "ceci") is replaced by an s if it is pronounced /s/, but not if it's between two vowels.
The letter t (as in "attention") is replaced by an s if it is pronounced /s/, which is doubled if it's between two vowels.
If it's preceeded by a c, it will be replaced by an x.
If the letter i is in the diphtongs ai (pronounced /e/), oi(pronounced /wa/) or ui (pronounced /ɥi/) it's replaced by a y, and if it's succeeded by a silent letter, it will disappear (unless the word has a homophone).
The circumflex accent is removed and words with it are spelled like in old french.
with a silent s after the vowels i and e and.
with a silent e before or after the vowel u.
It can also be removed after the vowels a and o
In the word "oignon" (where the i is silent) the i is removed.
Words that end in -ique now end in -ic
When the letter g is silent before an n, it gets removed.
If not, then the letter combination gn (pronounced /ɲ/) is replaced by ny.
The diaeresis (ë, ï) replaced by the letter y (which will be pronounced /j/) before the vowel that originally had the diaeresis.