you will have to learn this as you go with experience, but I would start with lowers, its pretty easy to identify little branches and leaves in the undergrowth that dont stand much of a chance of receiving light. as for the top, looks like you've got the space you need, start by removing a few large fans that are covering other tops, I would not do much once you're flowering, I can also say these plants are tough and you could remove a lot more than you think (80+%) and still overflow your space depending on the strain.. anyhow good luck, they look great, and you really can't fuck it up when it comes to defoliation.