Restoring photos with biro scribbled across faces/eyes is a difficult challenge, at least for me. I hope the faces in my attempt are still recognizable at a distance but I haven't been able to restore the original quality of the faces, at least not without spending much more time on it. Feel free to improve on this!
I started learning about photo restoration about 6 months back - with a lot of help from other restorers here. I don't have much time for it at the moment but I'm sure I'll pick it up again soon.
There's a Rootschat 'how to' subforum with useful tips and links at
http://www.rootschat.com/forum/resources-tips-tutorials/. It's also a good place to post any questions you have.
You can find (via Google) some useful video's on Youtube that show basic restoration techniques (patching, cloning, scratch/spot removal, etc.).
If you're willing to pay for tutorials, take a look at what Lynda.com has to offer:
https://www.lynda.com/Restoration-training-tutorials/1358-0.html?previousCategory=70You can subscribe to Lynda.com on a monthly basis.
There also some books (and e-books) on restoration. One of the most comprehensive is 'Digital Restoration from Start to Finish' by Ctein. It's 450 page step-by-step guide on pretty much all aspects of photo restoration. The author is a professional restorer and he/she goes into some 'advanced techniques' using selections, masks, filters, etc. to minimize the 'manual work'. It's probably best to be comfortable using the basic techniques before delving into the advanced ones!
But there's no substitute for 'learning by doing'. And for looking at the great results here and figuring out how they were achieved - and by asking the restorers. Different restorers work in different ways and most types of damage can be corrected using different tools. So there's never just one way of restoring a photo. It's good to try out different tools and learn what works best for in each situation.
While learning how to restore photos, I also learned a whole more about how to use Photoshop too!
Good luck!
Mike