Here's a list of tutorials which you might find helpful. Note: some of the information in these links is out of date and inaccurate or incomplete! Ask on the forum if you need help with anything.

Mapping Tutorials

D-Day mapping with QuArK
Mapping FAQ
Transparent fire mapping tutorial
Old list of D-Day pickups
QuArK barbed wire mapping tutorial

Map Guides

How to complete dday5 (archived link, no pictures)
How to complete invade5 (note: only noobs destroy the bridge first)
How to complete gbrsnow2

Modelling Tutorials

Modeling tutorials @ ddaynormandy.com
Milkshape 3D tutorials

Other Tutorials

Inside3D - Quake2
Milkshape 3D tutorials
Wally

1. Read the Game Rules!

2. Download the game and Install:

D-Day Full Install (211 Mb)

Download from mega.nz -

3. Click "Update Servers List.exe".

4. Launch the game using D-Day Launcher.exe, hit the escape key and join a server through the multiplayer menu.

That's it! To download more maps check out this thread.