snapmirror resync

If a snapmirror relationship is broken, data written to the destination that will need to replicated back to the original source do I just need to use “snapmirror resync –S original_dest:vol original_src:vol”??

Also, if I then want to re establish the origal src to dest relationship I do the same again in the reverse order?

Just want to be 100% sure I will not lose data!!

snapmirror resync

Correct.. just be careful which box you enter the resync command on   Resync will perform a volume snaprestore on the target to the last common snapmirror snapshot then roll forward the resync from there taking all changes that were made, then cut over and resync again the other way.  Often we have customers use a test volume and cut over, add files then cut back to have a comfort level with the procedure (and make sure no typos and document which system to type commands on).