Creating a bpm to connect multiple levels of Sales Order

Trying to figure out a way to create a bpm for a checkbox that i created and put on the Sales Order Entry Header tab. When checked I want it to check the Firm Release checkbox for every line associated with that SO#. Any ideas how to do that on bpm?

Almost every BPM example on this site would be a good place to start.
Basically, you will want to use a custom code block on a certain method and you will want to write a LINQ query to iterate through a record set associated with your sales order header, in this case you for each line, and perform the action you want to.