A basic understanding of rhythm is important when discussing time signatures and bar lines. Rhythm is the combination of short notes and long notes. Typically, a quarter note is said to get one beat, so it is a short note. A half note gets two beats and a whole note gets four beats. You will want to review all these notes, among others, and what they look like.
A time signature should be located at the beginning of a piece of music and features one number on top of another number. The top numbers tells you how many beats are in a measure (the space between bar lines) and the bottom number tells you what kind of note gets one beat. When “4” is the bottom number, a quarter note gets one beat. 2/4, 3/4, and 4/4 are typical and simple time signatures. Other time signatures, like 6/8, are compound time signatures, which have six eighth notes in each measure (or the equivalent of six eighth notes). Often in compound time signatures, the “beat” that is felt is a dotted note. For instance, in 6/8 time, there are usually two beats (two dotted quarter notes) felt in the measure, usually written as two groups of three eighth notes.
Bar lines are thin vertical lines placed in music to divide it into measures. A double bar line shows you the end of the piece of music. Music without bar lines is typically aleatoric or does not have a consistent beat or pulse. A double line (different from the double bar line in that the second line is not thicker than the first) shows a major change in the music such as a tempo or key change.
To put bar lines into music, you must first look at the time signature. If you have 4/4 time signature, there will be four beats in a measure, and a quarter note will get one beat. Count the notes until you reach four full beats, then put in a bar line. Continue this step until you have placed bar lines throughout the whole song. A double bar line should go at the end of the song. This process may be complicated if you have a pickup measure or upbeat, in which you have an incomplete measure at the beginning of a song. Double check your music to make sure each measure is complete.