Sometimes there isn't going to be a MUA and us models have to come with makeup. What advise would you give models that haven't got much experience really in doing their own makeup? (This goes for brands. techniques, must haves, styles...etc)

Have you tried the search  topic for this question?  I feel like this  question is beating a dead horse.  R-

I suppose a simple tip would be, less is more when it comes to eyes. You can always build the color and look but it takes so much time to correct or start over. Start with clean brushes. Unless the shoot calls for dramatic, a nice neutral eye is usually a good idea with a few individual lashes if you know how. A little bit of color on the cheek and lip is usually a good idea, too. Don't go crazy unless they want you to. Best thing to do is practice a few looks at home. YouTube like the commenter above said, is also good if you're not sure.

Foundation - be sure to use a photo-friendly foundation with no SPF. And, said above, talk to the photographer so everyone is on the same page with expectations for the shoot - have a Plan.

Not speaking to the OP in the sense that there is anything wrong with her skin.

But, really take care of your skin - see a dermatologist at appropriate intervals and follow their direction for the healthiest version of your skin you can have.

With great skin you can often get away with little or no foundation. Little bit of mascara, natural lipstick, and you're good to go.

Even if you have the luxury of a makeup artist, do this. Makes their job (not to mention the photographer's in post) so much easier.

I'm not a MUA, but I noticed a good pair of false lashes makes all the difference in the photos. Mascara just doesn't get the form and length right (and I naturally have super long lashes)

I was actually planning on becoming a MUA before I began modeling. I took the proper classes and everything. Waste of money it seems. Lol
I would personally recommend models take at least a free basic makeup course online, find the colors and styles that fit them best, and use that. A lot of it depends on the shoot you're doing. Some of my favorite, high pigment makeup products are actually just cheap drug store products I found on sale or clearance. Brand doesn't always matter.
Another thing to keep in mind is you need to check your favorite looks in different settings. That means florescent and natural lighting.
It also means checking how your foundation looks when using flash. Some foundations can actually look a completely different shade under flash. This is commonly caused by SPF in your foundation and can be highly problematic.