Once you have a stock, patched 4.08m game, and made a backup of it (thanks bf109boy) you can do:
OPTION A
1. Mod Activator:
(The link is inoperative)
2. 4.09_Mod_b1m:
(The link is inoperative)
This will give you some new planes, ability to add MODS, a modded 4.09b1m, and a switcher to switch between 4.08 and 4.09, but no new sounds.
OPTION B
1. ENG_Sound_mod:
(The link is inoperative)
(there are several versions of this, and after installing, you must install wrapper and a/c installer)
2. 4.09_Mod_b1m:
(The link is inoperative)
This will give you same as above, PLUS new sounds.
OPTION C (c for cnopicilin
)
1. Unified Installer v1.1:
(The link is inoperative)
(this is an eight part download .rar archive)
Do not click the "Download-here" link at the top of that thread,scroll down to find the 8 parts and instructions
2. Unified Installer patch v1:
(The link is inoperative)
This will give you everything in B, PLUS many more new planes and maps.
Whatever route you choose, please read carefully all instructions in the appropriate thread. I am just summarizing here.