Hi @megannqq thanks for posting in the Klaviyo community! It sounds like the best option for those types of emails would be a campaign. This is a one-off email sent to whoever you choose at the time and date you choose. 

Campaigns are different from flows in that flows are automatic and triggered by an action a user takes (like making a purchase, signing up to receive email, etc). 

This Klaviyo Community question goes through the differences nicely

Here’s how to get started creating a campaign.  You can create a segment of people who have gone through your welcome flow and send them the monthly newsletters as campaigns. 

If you’re looking to add all of the monthly newsletters to a flow over time, and have each person who completes the welcome flow go through all the monthly newsletters starting from the beginning, then I would create a new flow that is triggered by someone finishing the welcome flow and just keep adding your new monthly newsletters as they are created. 

Hope this helps!