It's helpful to chart the lines and syllables on graph paper to help you account for having the same number of syllables in each related rhyming line. What's important is to get the words to line up correctly so that the song sings easily.
Four ways of doing this....
1. Use the exact number of syllables per related line.
2. If you have fewer syllables in a line, use Melisma to fill syllable space.
3. If you have fewer syllables in a line, extend some syllables longer.
4. If you have fewer syllables in a line, leave a rest or two as fill.
#1 and #2 tend to work best.
#3 works good when you don't overdo it.
#4 can make the line choppy, but still is viable if used carefully.
Melisma is widely used in rock and pop music to solve this alignment problem and has the advantage that it adds mojo to the vocal arrangement. Melisma as a vocal technique is analogous to bending a guitar string up or down into a second or even third note. It's like doing string bends with your voice. It is less used in traditional folk music and some folk purists frown on it's use. But for rock/pop it can't be beat. Listen to old Beatles performances and you will find it being used in almost every one of their songs. It is one of the keys that made their music great. The Rolling Stones used it a lot as well.
To give you an idea, here's what wiki says about melisma. Melisma - Wikipedia, the free encyclopedia