Do you have any advice on drawing collars like on jackets and shirts ?

First off, make sure to get lots of references up! It’s hard to draw something if you don’t know what it looks like, after all.

Collars aren’t too difficult though, you just need to remember that they wrap all the way around the neck

So you might find it helpful to draw a couple elipses around the neck

And then build the collar up from that!

And this goes for any kind of collar

just adjust the elipses so that they match the kind of collar you want

And of course, this isn’t just for collars!