I just came back to learning Spanish and now see that each skill has 5 levels. Should I do all 5 levels in a skill before going to the next skill or should I finish level 2 in EVERY skill before going back to the beginning and working on level 3?
You will perish from boredom if you try to get to level 5 of each skill before moving on to the next skill. But doing one level at a time is not necessary either. After everything is at level 1 or higher, it's fine to just follow your interests, doing some "Practice" (what used to be called "Strengthen Skills") to counteract decay.