The Tavern

After reading the guide on species modding, it seems the core stuff is pretty clear. However, some things are still a mystery for me.
First of all - how do I add a description for my new race? Can I just paste it in alphabetic order into dat\descript\species?
Second - what to do if I don't have a corresponding monster for my species? And why do I need one? (What does the guide means "for display?")
And lastly - how do I designate tiles for my species? I want to reuse lava orc ones - what string is responsible for it?

Doesn't even have to be in alphabetical order. You'll probably also want to add an entry in docs/crawl_manual.rst.

tilepick-p.cc and rltiles/dc-player.txt are needed here.

More generally, the Faerie Dragon species https://crawl.develz.org/tavern/viewtopic.php?f=8&t=25195 is fairly complete - do everything that does, and you'll have done everything you need. That's probably how to answer your second question, too.