Here's a quick overview of how to show the length of a sound using the IPA:
I also get into what tends to make a vowel longer and what tends to make it shorter:
Usually tense vowels are longer, and lax vowels are shorter. Similarly, vowels before voiced consonants or at the end of a word tend to be longer, and vowels before unvoiced consonants tend to be shorter because the voicing wants to cut off to prepare for that consonant. You'll learn the symbols for lengthening and shortening a sound.