Disclaimer - (The following DIY project is purely “at your own risk”. As with any DIY project, unfamiliarity with the tools and process can be dangerous. Posts should be construed as theoretical advice only.
If you are at all uncomfortable or inexperienced working on these projects (especially but not limited to electronics and mechanical), please reconsider doing the job yourself. It is very possible on any DIY project to damage your ride, void your warranty, create a hazardous condition, or harm or even kill yourself or others.)
First things first - disconnect your battery!
Read through the installation manual that accompanies your particular model of stereo head unit and get familiarized with the wire schematic.
Next thing to do is to fit your new stereo head unit to the new double DIN enclosure. Some filing may be necessary on the inside corners, especially if you have a motorized face plate. Mount your stereo in the enclosure and verify the splash guard closes fully and does not hit the face of your head unit:
Next you will want to remove the upper dash and trim parts. I won't detail this as it has already been covered in another DIY thread. So, here is a link to cover that procedure:
Start sketching out your cut lines on the aluminum. The bottom will start a 1/4" above the cutouts for your toggle switches. This will make sure that the splash cover does not hit the dash top covers and the head unit does not bottom out in the lower dash. You will cut through the thin aluminum plate and the plastic of the upper dash. Be very careful with the aluminum as it bends extremely easy and likes to lift away from the dash. Fit the double DIN in to your upper dash to test fit.
Next you will want to lay your dash back in place. Put the push pins back in place to hold the upper dash in place. You will notice that the remaining lower dash plastic will now need to be trimmed as well to match up with the new opening you made in the upper dash. Now you can go ahead and remove the upper dash again to make the installation of the splash cover to the lower dash plastic trim. Lay the splash guard in the section that you just trimmed out of the lower dash with the mounting holes on the engine side of the dash. Mark your holes and drill them for your mounting hardware. I recommend using these T-nuts:
They can be found at your local hardware/big box stores.