The baal koray must review the sidra until he is fluent with all of the particular accents of syllables. He should read deliberately and not quickly lest he swallow a letter or word without pronouncing it. (MB 6)
There are two categories of mispronunciation:
A change in vowelization or trop was made which changes the meaning – have the baal koray go back, fix the mistake, and continue again in order.
A change in vowelization and trop that does not change the meaning – the baal koray does not need to go back but we reprimand him. (ShA 1 – MB 2, 4 – BeH “aval”)
Deleting or adding on a letter to a word also depends: if the meaning is changed go back – if the meaning does not change do not go back. (ShA 1 – MB 1)
Deleting a word requires you to go back, even if it doesn’t change the meaning. (BeH “oval”)
If no one in the town knows how to read without mistakes and no one knows how to correct the baal koray, perform the krias haTorah without the berachos and omit the haftara. (MB 7)
If someone knows how to read but without the trop, perform the krias haTorah as such with the berachos and haftara. (ShA 2 – MB 7)
Even better would be if someone knows the trop by reading the sidra from a chumash – he should chant the kria in a low voice in front of the baal koray to lead him in the correct vowelization and trop.(MB 8)