Your letter should have a single focus. You might have to write more than one letter. The first thing to do is pick a topic. It might be someone who has disappointed you, or something you're not pleased with, but you need to know why. We have to drive our points home.
The ACLU offers good insights into writing elected representatives on their website. Wander over and have a read.
Tell the recipient why you are writing. What did they do? Or not do? Paint a picture here. Supply examples of better behavior.
Here is where the voting records might be useful. You might also want to look at polling statistics, inflation numbers, unemployment figures, environmental protection figures (or lack thereof), education statistics, local land valuations, etc. Some good resources include:
https://omaha.com/ (Paywall)
https://journalstar.com/ (Paywall)
(Hot tip: Putting a difficult website into Google Translate Websites or archive.is usually makes them more cooperative. I'm looking at you, local newspapers.)
Not that facts matter to certain folks, but if you're feeling especially ambitious, access these resources for peer reviewed journal articles: