^ Those points of "advice" are so general and devoid of details that they are completely useless. I have to wonder why you bothered writing anything at all.
Keeping frequencies and things like that "even" is a persistent challenge in audio that is pretty much the entirety of mixing in the first place. By NOW I've been doing this long enough and have gotten good enough (especially compared to my first Ocremix here) that I can throw around some weight in authority on the subject, while still "hillbillying" my way through it since I still lack formal training beyond spending 15 years trying to get better obsessively. That is the pretext for me telling you what I do to achieve results:
1. EQ can be simplified here (in my own personal, unpatented and unofficial model) as three things: The Floor (the bass), the Body (the mid) and The Voice (the highs). The bass literally brings to mind a stage floor for the performance on the stage. The Body is the "stage presence" and gives the soundscape a kind of "muscle" to the overall sound. Take down a whole bunch of the mid in your EQ to hear what I'm talking about. The Voice is rather obvious: above the midpoint is where most melody and leading sounds come from above the floor and body.
You'd think "mud" would come from the floor, but I find it usually comes from the body below the midpoint. When I'm cutting "mud" from the mix, it's between 100 and 500. Additionally, I find a lot of instruments that come up in DAW are already very bass heavy and muddy. Sixto Sounds used to make fun of me for this, but I also recommend cutting some bass out of the bass instruments themselves. As silly as that might sound, you just need to do it sometimes to achieve the right balance.
2. I never learned how to use Reverb properly and I still have trouble with it, but there are some solid fundamentals I can pass on.
2a: Find the amount of reverb or general sound you want, then go straight to DRY. Then add WET (i.e. adding reverb amount back in) slightly and slowly until it sounds like you're achieving a balance that isn't too echoey or out of touch with everything else while always opting for it sounding too dry than too wet. I only had one college class for audio production and it wasn't even the right class, but my professor said something that (pun intended) resonates today: "You're supposed to make reverb sound like you're not using reverb at all". He was a goofy-ass guy, but this was spot-on.
2b: Not all reverb plugins are equal. While there typically is no such thing as a bad reverb plugin so long as it provides any reverb at all, not all reverb plugins and sounds work for everyone. If you're getting too much mud and slush from your reverb stuff, you might need to go looking for a reverb that does sound like what you want.
2c: For instrument channels that you're using reverb with, a delay effect should be on as well. The trick here is to put it on a very basic 3-step delay, then turn it down to like 3% or 5% where you can't hear the instrument bouncing off the sides (unless, of course, you want it to be bouncing echoes like many instruments in modern productions do). This is a trick to finalize wetness that I learned from Rozovian, who in turn learned it from bLiNd, IIRC. I use a delay effect on practically every channel except the main bus (main channel for all instruments).
2d: Some reverb effect plugins let you EQ the reverb itself, but I find this effect is much more subtle than it might sound. If reverb is sounding too muddy or slushy, you need to fix it in the instrument channel, not the reverb plugin. The reverb plugin is just for minimal tweaking after the real work in the instrument channel.
3. In heavy, thick productions, the bass instruments should have their stereo separation reduced to straight up MONO and then very slightly adding some stereo back in, or not at all. Between the bass and drums, it's the drums that need stereo placement and width and keep them wide while focusing on a narrow bass will fit the floor correctly. I also add a compressor on the bass. I use FL Studio 11 and I just use the FL Studio Multiband Compressor on "Mastering 2.4db" preset, then style the EQ and mixing channel volume as needed. I read this bass tip in some random audio mixing book years ago at a Booksamillion and it has worked ever since.
4. Checking to see which instruments/frequencies are wrong during the song production is just a case of experience with successfully doing it. You won't really know what to fix until after you've already done it right a few dozen times. If I have trouble with this, I just mute and single out the instruments starting from the bass up (or wherever it sounds like the problem is) and automate volume or EQ as needed until it no longer makes me think something is wrong with it.
4a: This deserves it's own line, but learning how to automate the volume levels and EQ frequencies in your DAW is essential. It's complicated and a pain in the ass to do, but it's easier than other hillbilly methods of fixing stuff. If I had learned to do that in my early days, I would saved myself dozens of song headaches.
4b: Also remember that most instruments and frequencies are not supposed to have the same volume and intensity all throughout the song. Sometimes the bass is supposed to be pulled back, sometimes the piano or guitar have to become inaudible, etc. Sometimes being uneven is what makes it sound organic and makes it work.
And that's all I feel like typing. Enjoy.