There is a sebderm subforum on Reddit. It's been really helpful for me. When mine was at its worst it looked very very red and inflamed, like rosacea. I would post a picture from ages ago before I researched how to improve it but it's so awful it's embarrassing!
I followed an anti inflammatory diet for 5 days which really settled it down (this cut out gluten and dairy completely). I've discovered with trial and error that gluten isn't great for me - honestly it gave me terrible wind, looking back. It's not that I'm coelic, I just think I'm sensitive to it. I try to avoid it but will have pasta or pizza occasionally, and always find I have bloating and wind afterwards, and my skin starts to become inflamed too.
On my head, I used Selsun - you probably have to ask a chemist for it. It stinks and is very drying on the hair. But it worked for me. I've run out so not for a while. Nizoral (ketoconazole) did nothing for me. But the Selsun Iwith selenium sulphide) worked well. I used to actually put a bit around my ears, forehead and round my mouth creases too. while washing my hair and found it helped.
As for cleansing my face I use Garnier micellar water for sensitive skin, to remove makeup (clear bottle with pink cap). Then I like to use Clinique Take the DAy off balm afterwards and wipe it off with a set muslin then give my face a good rinse. I found any soaps or foaming cleansers just far too harsh, even though my skin seems to produce a lot of oil over the day, it's like the skin barrier is impaired, so gentle is the way to go. I then spray my face with Avene spring water and while my skin is still dampish I use some cream called Seblair (active ingredient is piroctone olamine, same as in some Head and Shoulders anti dandruff shampoo)= I get it mail order from a company called Dermacare Direct. They also do a shampoo in the same range but it's a tiny bottle and I didn't find it as helpful as the Selsun so I haven't bothered getting the shampoo again. But the cream has been great - so calming.
Re foundation, I used to use Clinique Superbalanced makeup. But when my seb derm was at its worst it just relaly seemed to not tolerate it. So now I use a CC cream instead - La Roche Posay Rosaliac CC cream. It's less coverage than a foundation and only one shade so slightly orangey but it does kind of blend in. I've got used to having the lighter coverage than a foundation now.
Sunlight is supposed to be good for seb derm. Honey masks are supposed to be fantastic (google), yo'ure supposed to leave it on for a few hours and I find that aibt awkward so I don't do it very often but when I do it does make a difference. Also tea tree oil (diluted in a bit of carrier oil) is supposed to be good too, and I have used that, but, as with the honey, I forget to do it of an evening as I'm busy. I've heard aloe vera is good too but I haven't tried that.
You need to be careful with any oils either on the face, or nutritionally. Some are beneficial and will improve the seb derm and skin barrier and some will just feed it and make it worse. Omega 3 is good, so I take those in capsule format, plus a high dose vitamin D and a multivitamin.
Reduce the gluten, definitely, I'd say. Hard for teens, I know, as they like a lot of pizza and pasta usually. I now have porridge with berries & sundried apricots for breakfast (I've gott used to it with no milk or sugar - just water), usually a salad for lunch (I use tinned mackerel and do a home made dressing with olive oil/lemon or white wine vinegar). Basically foods that are anti inflammatory will help. Plus keep hydrated - 2 litres of water a day if poss. (not that I manage this half the time ha ha but when I first was trying to get it under control I stuck religiously to that rule and it did help loads). Cut right down on bad things like dairy fats, and sugar. Again, hard for teens, that, I know.
It's a chronic condition, unfortunately, so will just come back if you don't try to keep it under control. My 15 year old son seems to be developing it now too. Think he has the same skin barrier issues as me.