Shoulder Mobility Plan?

Hey everyone

Posting this here as google is a minefield and I’m hoping to get some answer from experienced lifters/pros who know something about this particular issue.

Basically what I want to know is what to do for shoulder mobility. For example, when hitting the back double biceps pose I can’t comfortably get my arms pinned back like they should be, also feels like I can’t get them to rotate back as much as they should to really open everything up.

Also I know that once I have good shoulder mobility it will increase my range of motion on back day.

Thanks guys!