Sorry, not an expert can't reply.
Only kidding
Some advice:
*Have a plan
*Do a rough sketch on grid paper
*Split the map up into 'parts'. This way you can compile small areas. This makes it easier to target errors and will prevent you from going insane when you compile a massive map thats has leaks and errors all over it.
*Carve is a no-no most of the time. Use Vertex Manipulation, you will love it!
*Be realistic. Half-Life is over 5 years old, so don't try to make a 'super duper, ultra fantastic, mega gigantic death map' because there's no chance that it will work. Work to Half-Life's limits.
*Having said that, theres a lot of things you can do in Half-Life. Be creative. No one wants to see a grey box with a gun and a few million enemies
*Have fun!