English metre works on beats or accents rather than syllables, you can put extra syllables in often. The thing is, say it out loud and listen to yourself saying it.

One trick is to use an existing tune to get the metre ironed out, and then make up a new one to fit the lines. That's in the case where the tune doesn't come along with the lines.

And another trick with a song with a complicated idea to get across is to write it out in a prose form, to get the idea clear, and the proportion between the different parts. And then turn it into a metric form, and maybe to work in a rhyming scheme. (Though rhymes aren't as essential in a song as people tend to assume. Getting it to scan to fit the tune is what really matters. There are lots of good songs which do completely without rhyme.)