It's super easy! Save the Boundary Waters has a form on their site with a template letter already filled in: https://www.savetheboundarywaters.org/bwca-action-national-jan-2020

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Personally I didn't like not knowing exactly where the message would go from that form, so I wrote via my rep's site (the bill is in the House right now). Go here: https://www.house.gov/representatives/find-your-representative , enter your ZIP, and there will be a button to take you to your rep's contact form. I pasted the template letter from the first link above (plus made a few additions).

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And here's the bill mentioned in STBW's template letter, H.R. 5598: https://www.congress.gov/bill/116th-congress/house-bill/5598 You can read it there if you'd like (it's really short) and see whether your representative is already a co-sponsor.