Welcome to the Forum! Glad you're enjoying the LG Bible!
The reason that it is more efficient to have R S T along the side of your diagram is that it makes your base more fixed. If you just leave it open as in the first diagram, it becomes much trickier to diagram the first two rules because you don't know where the girls are. You know that each girl is going to be in each dance so if you just go ahead and set them in specific slots, it becomes easier to diagram where the boys go based on who they are partnering with.
You also may want to check out the lengthy discussion of this game that we have elsewhere on the forum: viewtopic.php?f=412&t=2214
Games are definitely something that becomes easier with practice. The more games that you are exposed to, the more you'll start to see the same types of rules, inferences, and setups and the more confident you'll become with setting your bases and moving through games quickly. But there are always games that are a little stranger, and therefore often tougher, than others. Pattern games are rarer than other types so they can be tricky at first. But keep at it and you'll see improvement!
Here are some more tips for practicing games: https://www.powerscore.com/lsat/help/ls ... e-tips.cfm
Hope this helps!