Rule no. 1 - Building muscle is NOT complicated
Get stronger and stronger in a medium rep range (6-12 reps), especially in the multi-joint movements. If your muscles aren't getting bigger, stop looking at the latest article talking about "muscle fibre types" and actually add some weight to that 150lb squat
Don't obsess over routines.
Don't over-think how muscle is built
Don't look for the "newest" "coolest" method of building muscle (be wary of most articles)
Don't under-estimate your potential strength/size gains; aim high!
Don't try to get big and lean at the same time (focus on getting bigger 1st with moderate fat gains)...the more muscle mass you have, so long as you didn't screw up your insulin response (usually via too high carbs/not enough carbs) and get "too fat" with a crap routine, the easier it is to cut.
Don't zig zag; that is, don't bulk for two months, and cut the next minute. Make your mind up, sort your diet out (not too many carbs/fat, have plenty protein), and get stronger each week. You can re-comp (if fat levels get out of control, e.g. 18%+ bodyfat), but don't go into a full blown "get ripped" diet, you'll loose your strength gains/muscle if taken too far.
DO follow those who've already been there and done that.
Be consistent with your routine. Find a half decent one and stick to it (a decent one has you hitting muscle groups twice every 5-8 days, and doesn't over-do the exercise selection). If you're not getting the results you were looking for, it's likely lack of rest/calories that's the problem, not the routine (unless it was really bad to start with).
Eat enough (including protein/total calories). If you struggle to eat enough, increase in fatty foods a little (e.g. beef/oil/eggs/nuts etc). Incidentally, increasing your protein foods will increase calories via more fat too, so you can't go wrong with increasing protein.
Avoid very low carb diets (long term ones especially), you'll just get fat/flat/weak (carbs are NOT evil). Adequate carbs are essential for good performance, hormone balance...which overall effects muscle gain and fat burning etc.
Take rests when appropriate. If you can no longer push as hard, or feel as motivated as before (you should feel relatively psyched before lifting) then you need to go easier for a few weeks in order to 'recharge' (reduce load somewhat, skip a few workouts etc)...if this happens too often (like your strength keeps going up and down), then you need to either increase calories or re-asses your routine if it's much different from 'normal' (e.g. too many exercises especially big ones, too many sets, frequency too high/low etc).