I installed new 6x8's in the front doors of my '04 ZX3, and I used the foam baffles from Crutchfield.com
There are two ways to do this, and I picked the hard way, I'll give you the easier way at the end.
There are other threads for removing door panels, so I'll leave that part out
For this version you'll need duck tape, longer screws, washers (if the screw heads aren't big enough), scissors, and your speaker's spacer (shown here)
IMPORTANT! Roll down the window before you start or you won't know how deep the baffles can go!
Here are the speaker, crossover, and baffle I used
Here you see how the baffles are too deep to fit the front door properly
Cut the top off the baffle so there's about 3/4" protruding
Use your scissors and make cuts in the top of the foam approximately 3/4" long about every 1" -1 1/2", then slide the spacer up the baffle and use strips of duck tape to fold the foam over the spacer
Finished it should look something like this
Now stick it in your door
And install the speaker with the longer screws and the washers.
I didn't want to punch a hole in the baffle, so I ran the wires out the front between the speaker and the baffle. The foam is thick enough that the wires don't create a gap.
For the easier way:
If you don't want/don't need to use the spacer, go ahead and do the cuts in the foam and then tape the flaps to the door panel. Then just install the speaker as normal.
If you're curious how I attached the crossover:
I made sure there would be enough space for it with the trim panel in place, then I checked behind the steel panel and made sure there was nothing in the way for drilling. Then I drilled holes and used zip-ties to secure the crossover. When drilling each hole you'll have to stop part way and remove the gorilla snot from the drill bit.
If you have manual roll up windows, here's a tip to make your life easier when re-installing the trim panel:
Put the clip back in the handle before you put the handle in place, then just snap the handle on. You don't have to squeeze the clip on through that tiny gap with the handle already on the door!