You can learn a lot of useful things in art school if you pay attention. You can learn the same things elsewhere but I learned them in art school.
One thing you learn if you take painting is that it's generally best to work the whole surface of the painting at once rather than concentrate on one area then move to another area.
You're painting a portrait and you really like the model's nose. You want to get that nose just right. DON'T START WITH THE NOSE. If you start with the nose you won't leave room for the chin or the top of the head or something. You're supposed to sketch in the whole composition first so you know from the beginning that everything you want to include will fit in the picture.
Then you put in the large areas of color, work with the large masses of the form. Don't forget the background - that counts too. Now you can work on the nose. BUT DON'T JUST WORK ON THE NOSE. Because while you're working on the nose it will become disconnected from the cheeks and the chin will float off to Hawaii. What are those two small blobs up there? Those are supposed to be eyes? Nuh-uh.
But the nose is really good. But I'm out of nose color so I have to mix more for the forehead. Wait, that color isn't working at all. And I just noticed that the eyes are in slightly the wrong place and I should move them both over a little to the left and shift the angle of their axis a little down. But then the nose will be where the lips are supposed to go! Maybe if I can use the nose as a pivot point I can move the chin a tiny bit over here... But now the temples are all messed up. Fuck! Now what do I do? I'll have to start over!
Yes, you'll have to start over. And that, my friend, is painting.
But the nose was perfect! Are you painting a nose or a face? Do you want a painting of a beautiful nose floating in a sea of mushy chaos? Because if you want that that's fine.
In the olden days popular painters had ateliers full of apprentices who specialized in noses or lace or backgrounds or helmets or whatever. They probably got some kind of room and board for their labors. Maybe. Those days are over. Being the finest painter of noses in the world is a wonderful personal accomplishment but you probably won't be able to eat because of it.
If you want to paint a portrait that looks like a whole human head (or dog head or fish head or whatever your subject is) you're more likely to be successful if you do a dab on the nose, then the cheekbones, then the forehead, then the hair, then the chin, then maybe back to the cheeks for a stroke or two then the light in the background then the chin again... See what I mean? Bring the whole thing along little by little so you know where everything is and they're all developing in relation to each other and you don't get lost.
You're still going to have to start all over again a few times because that's just how it goes. Something will inevitably stand out and look great but it will be too soon so you'll have to get rid of it. If you find an area you want to preserve that's a danger zone. Mess it up - unless you're almost done and the other areas are almost there too.
Paintings go through a lot of stages before they're finished. You don't just start with nothing and then it gets better and better. Sometimes you start and it looks very promising. Then you realize it's not going to get where you need it to go that way and you have to mess it up to make it better. It looks great then it looks like unsalvageable crap then it looks kinda better then it looks pathetic then you figure out what it needs and it's done. Except for that one little part over there...
Screw that part. It's done. If you work on it any more you'll be working on it forever and you'll never finish it. Sometimes you just have to call it done. Let it go. That's a very freeing feeling.
Of course when someone says "you're supposed to..." that means that someone else's opinion of what you're doing matters. The truth is you can do whatever you want. Particularly in art. If you get the result you wanted then you went about it the right way. The ends justify the means in art as long as you're happy.
You don't really have to sketch out the whole composition in the beginning if you don't want to. Messed-up, out of whack portraits can be kind of cool. Sounds like a profitable exhibition if you ask me. Messed-up is very popular in the art world these days. It "reflects the chaotic nature of the times and the artist's disassociation from society" or some shit or other. If you can explain it to somebody you can make it work.
I suppose knowing how to do it "right" is a tool to have in your kit to be used if you need it. You never have to use your tools if you don't want to.
I'm calling this done. Later!