The basic features for formal writing-
Write all verbs in full. Do not use contracted forms like don’t or can’t.
Mark your vocabulary. Certain words are considered to be informal. Examples- fix, start, OK, thanks etc. Avoid them in formal writing. Instead,use words like repair (for fix); commence(for start); all right(for OK)and thank you (for thanks).
Limit the usage of phrasal verbs. If avoiding is difficult we can limit the use of phrasal verbs. It is usually possible to express the same idea using the standard verb.
Do not use abbreviations such as info ( for information) and asap (for as soon as possible).
Limit the usage of Passive voice.
Avoid informal intensifiers like so and really. Instead, use more sophisticated ones like extremely, highly, entirely etc.