Double the final consonant before a suffix beginning with a vowel if both of the following are true: the consonant ends a stressed syllable or a one-syllable word, and the consonant is preceded by a single vowel:

drag becomes dragged

wet becomes wetter

occur becomes occurred, occurring

refer becomes referral, referring

Do not be tricked! Not all words take a double consonant. As is noted above, it is all about where the stress is placed. For example:

Develop becomes developed (not developped)

The reason we do not double the "p" is that the stress is on the "e," not the "o."