Sea Shepherd has done a great job and put some checklists and to-dos together, as well the marine tracker form to take part on international marine debris tracker analysis.

https://www.sea-shepherd.de/debris/index.php/recources

      How to start:

• check clean ups and get listed
• get familiar with the area you want to clean up upfront
• tell the local trash management that you are planning to do the clean up
• tell the local news about it
• get enough gloves, bags to collect the trash
• find volunteers in social areas (schools, but as well companies)
• tell the volunteers upfront why you are doing this 
• near coastal areas or other water areas make sure that everyone is save and nobody gets to close to the water (especially kids!)
• separate the trash you have found together with the group and analyse the trash - there are always curiosities which are fun to share with all
• summarise with all the catch of the day, ask about the main polluter (cigaret butt, pet bottles, ...) and discuss mitigation strategies, how you can start to live a plastic free live (you can find a lot here on my website) 

• as I support Sea Shepherd it make sense that we all use the same forms and lists to clean up the planet, they have as well a huge volunteer network and sometimes it make sense to organise it together :)