While they can be challenging, painting a portrait is not actually different than other subjects. You still have to see shapes and light and shadows.
The face is made up of various planes. So get an idea of the general shapes first before adding details. If the proportions start looking right before you add the details, then you can go forward. But continue to check proportions throughout.
Because portraits often take a fair amount of time to paint you have to be careful of seeing things other than as they really are. The longer you work on a painting the more you start to see it as right, even when it is not.
Flipping the painting horizontally can help with this. You can keep two copies of the reference, flipped different directions, so that you can work on the painting with both orientations.
Taking frequent breaks can help in seeing better as well.

Below are videos showing how to find the proportions of the head. This can sometimes help when making a portrait. Most heads follow these patterns.
For those just starting this video by Yupari Artist can help you understand painting a portrait.
https://www.youtube.com/watch?v=2wltLvLz8gI

This is a more advanced video by Bradwynn Jones on head proportions and feature placement using the Reilly Rythm method.
https://www.youtube.com/watch?v=Isioa6-3R34&t=5s

An interesting video on nose anatomy from the same artist:
https://www.youtube.com/watch?v=3e6VSPJvBNY&t=449s

Eye anatomy:

https://www.youtube.com/watch?v=Y-AkTBtydOc

Learning more about the anatomy of the bones, fat pads, tendons, etc. in the face can help with your painting.

There are "zones" of the face that tend to lean towards a certain color.
Generally the bottom of the head around the jaw has more green or blue. The middle of the face has more red, due to more blood in that area, and the top of the head tends to have more yellow.
This is subtle, but can be seen in many paintings.
This blog post by James Gurney gives more explanation and examples:
http://gurneyjourney.blogspot.com/2008/05/color-zones-of-face.html

This art station tool helps you to see the planes of the head in different lighting.
https://www.artstation.com/artwork/GX3Ax1

Sometimes it can help to have your reference upside down and paint upside down, then flip the image vertically in colors. The reason for this is that our brain tries to impose what it thinks of as a face onto what you are painting. By turning it upside down you see the shapes and shadows with less interprestation by your brain.

The tips tutorial for using reference also applies. If you have not yet checked it out click here:
https://www.colorslive.com/tag/referencetips

I'd known about flipping the painting horizontally or vertically to keep a fresh perspective, but actually keeping flipped references to work with, as well, is actually a revelation to me, hahah! Seems like a simple idea, but that thought just blew my mind right now. x'D
Thank you very much for these tips. Perhaps I will attempt this contest! :D (...Maybe. :'D )

The lighting on that portrait is incredible!

@TB_Studios it was challenging, I usually don't do top-down lighting like that. I struggled with some of the proportions on this one, but I enjoyed doing a less blended, more tile based approach.

@Taco-San yeah it occurred to me one day to do it that way, and it worked well. So I figured it might help someone else.

You did wonderfuly!

Bro, you've outdone yourself!!!

I really like the lighting on his skin in that picture. For me, the best way to tackle portraits was by starting basic. I did greyscale first to only focus on proportions and didn't try different angles until I had the head-on view sorted first

@mcollins I agree that greyscale is definitely best for starting out. And some can look very striking.

Question: what is the best brush to use for relistic stuff?
Sorry im honestly trying to improve my relistic DrawingX'D

@Artyeevee If you are just starting out it may be best to use the hard round brush. When you get a bit more comfortable the bristle brush allows for more textures, but takes some practice. Don't use the bristle brush with too much transparency or it can get blurry, which happens to me at times.
Try to use the largest brush size you can for a given line, only making it smaller when needed.

Ok, thanks:D

@Artyeevee For the painting in this tips tutorial I used the bristle brush.
If you click on this tag you will see one done with only the hard round brush:
#t73hd

And this one was done with only the soft brush:
#t73soft

The all soft brush painting gets blurry in a hurry, so I would recommend the hard round to start with. You can still get textures with it if you practice.

i cant seem to find the Hard round brush?

Also thanks for your tips they really are helping!
;D

@Artyeevee Sorry, using old terminology. In the Switch version it is called the "Classic Hard" brush.

Another tip when drawing a portrait is to upload it to a large screen on private in the gallery, and then take a day or so to look at it before uploading it so you see mistakes. I always get impatient and skip that step., but it helps to find errors.
See my gallery for a version of this portrait that fixed a couple of things.

Thank you!

Something to keep in mind when you are painting celebrity portraits: make sure that it shows the features that the person is best known for.
If the person usually has a particular haircut, but for some reason did not in that photo, you can still paint it, but it will be harder to convey who it is to others. It might be best to pick something that better matches people's mental image and expectations.

As a general rule portraits for males can be easier to paint than portraits of females. This is because women's faces tend to have more subtle transitions between planes, while men's have more defined shifts in values.

Be careful picking reference with very diffused lighting. It can give a soft, subtle look, but it can also be harder to portray the 3d form. Stronger, more dramatic, light makes it easier to show the form.
However, really strong shadows can also make someone look older, as planes can harshly contrast, wrinkles and furrows show up starkly, and highlights are more pronounced. So if you are painting a younger female you have to be more careful with really strong light.

https://www.youtube.com/watch?v=gpMXNqEwj7E&t=312s

This video explains how understanding the planes of the face can help paint a portrait.

A bit of portrait inspiration:
https://www.youtube.com/watch?v=VlJHE7PQfIU&t=616s

Sometimes it helps to use a grid to see the proportions accurately. you put a grid on your reference, and then a grid on a layer on your device (the pixel brush helps with this on the Switch or Beta). Then as you draw you can see how the elements being drawn match up to the grid. Here is a video demonstration:
https://www.youtube.com/watch?v=BFGqgdSTLiM

Fun fact, Albrecht Dürer (1471 –1528) came up with a couple of ways to use a grid method when drawing form life. One of these took a wooden frame with strings crossing it that he would view the model through!

very cool!

Sooooo much reading my brain hurt >-<

Wow, That portrait of Rocky Is Amazing! You've totally nailed it! So Professional!

that looks like ramboo

this helped a lot. thanks

that not a tutorial

rocky !! :D it's really good, you captured all the facial features incredibly and the purportions are perfect!!!

how bout explain in vid

mind blown