Try to keep your letter between 100 and 250 words. An editorial too long will not be finished by most readers, especially if they don't agree with you. Keep your letter focused on a single topic and try not to ramble from subject to subject. Focus on a single proposed bill and state the bill number, that will usually get the readers attention. Plus stating the bill number lends some credence and weight to your letter.
Most newspapers have a section set aside for readers letters.
Write the name of this section on the outside of your envelope. It might get your letter into the paper a day earlier. Make sure your spelling and punctuation are flawless before sending it. If the newspaper staff has to rewrite your letter to make it readable, it will not be published. So do it right or don't do it at all.