There are heaps of walk throughs on how to sound deaden doors, so I will only cover a few tips I have picked up along the way which I have not/rarely seen posted here and other sites.
1. Good preparation is the most important part, if you skimp to save time here you will have to redo everything and buy new sound deadening, when it starts peeling off everywhere. K&H wax and grease remover gets everything off easily with no water required and takes just a wipe with clean rag to take off the minimal residue.
2. You do not need to cover every square inch of every panel; we're not building a dam here. As long as you get at least 50 to 75% of the panel covered you are adding sufficient weight to the panel to stop it resonating at common frequencies. Spread the pieces of sound deadener apart if you are not covering much of the panel. I always ensure the area directly around where the speaker is, is well deadened since that is where the energy level is highest.
3. If you want to seal up your access holes, I have found the best thing to use is foam core sheet which is readily available from Riot Art. Cut it to size of openings, then fit it in place with some duct tape before laying the sound deadener on top. I have tried MDF before but it just soaks up water, swells then goes mouldy.
4. Sound deaden the inside of your door trim too; it is thin plastic which resonates even more easily than thin metal. The most important area is just above where the speaker is.
5. Use adhesive weather seal strips around the perimeter of baffles and speakers to ensure a good seal since most surfaces are not 100% flat. Allows easy speaker removal later, unlike gasket sealant (that sticky goop stuff)
6. After building many sets of baffles using MDF, ply, MDF coated in enamel etc; they have all soaked up water, swelled, then fallen apart. I have since started using plastic sheeting. Since it is not easy to buy unless you get in bulk huge sheets, I have found that plastic kitchen chopping boards are perfect. They are around 9mm thick and very easy to cut with a drill and jigsaw. $5.00 each from the Reject Shop.
With my speakers which are 59mm deep, using this sheet as a baffle I still have just over 5mm clearance before the door glass mechanism.
Use self tapping metal screws in this plastic sheet; the stainless steel type.
7. The tweeter in our cars is easy; just unscrew the hold one from the plastic mount then the new one is easily secured into it using a hot glue gun.
8. Use a rubber hand roller (also called "brayers") to press the sound deadener into the door otherwise you're gonna have really really sore thumbs if you do it all by hand. Also from Riot Art around $12.00 for the smallest one.
9. If you accidentally sound deaden over holes which the door clips fit into, don't just poke a hole through the sound deadener since the clip will then not sit flush against the door, and your trim will now have a gap in that area. Cut the deadener *around* the hole at least 5mm perimeter.
That's about all I can think of; if anyone else has any others post away