Writing to peers is a bit of a faff. Well, the writing bit less so, working out who to write to is the faff.
http://www.stilleu.uk/lobby-lords-brexit-bill/ is the best source of suggestions for what to write. It also has information on how to choose who to write to.
Write to a specific lord (search by name) here:
https://www.theyworkforyou.com/peers/Or use
https://www.writetothem.com/lords to search by other factors - e.g. words they have mentioned in debate, or connections to a location. The other benefit of this is that you can check whether a given peer is someone who turns up to debates or not.
These both limit you to 6 emails a day (the rationale being that above that, messages will be dismissed as spam). Many peers use the general House of Commons email address and this is a convenient way to email them. The form will put in the correct form of address for you.
However, using
https://www.theyworkforyou.com/peers/ you can look up email addresses, and if you find peers listing a different address, not just the HoL one, you can email them separately at that address.
http://www.parliament.uk/business/lords/whos-in-the-house-of-lords/how-to-address-a-lord/ for how to get the form of address right (NB that's Dear Lady Mobarik, frex, not Dear Lady Baroness Mobarik, not completely clear from that article).
In addition, the following are members of the government so subject to whip, and so may be less useful to write to:
Baroness Neville-Rolfe (Treasury), Baroness Williams of Trafford (HO), Baroness Joanna Shields (HO), Baroness Anelay of St Johns (FCO), Lord Keen of Elie (MoJ), Lord Nash (Education), Lord Bridges of Headley (ExEU), Lord Prior of Brampton (BIS), Lord O'Shaughnessy (Health), Lord Henley (DWP), Lord Ahmad of Wimbledon (Transport), Lord Bourne of Aberystwyth (DCLG), Baroness Evans of Bowes Park, Lord Dunlop (Scotland), Lord Gardiner of Kimble (Biosecurity), Lord Bates (int dev), Lord Ashton of Hyde (cult), Lord Taylor of Holbeach, Earl of Courtown, Baroness Mobarik, Viscount Younger of Leckie, Lord Young of Cookham, Baroness Goldie, Baroness Buscombe, Baroness Vere of Norbiton, Lord O'Shaughnessy (all whips)
Other useful links:
https://www.crowdjustice.org/case/parliament-take-control/ "Case updates" has ideas and material they've sent to all the Lords and previous stuff they sent to MPs
https://www.bindmans.com/uploads/files/documents/UK_Parliament_EU_Citizenship_rights_booklet.pdfhttp://www.crossbenchpeers.org.uk/interests.htmlhttp://lordsoftheblog.net/
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