Follow the instructions on the iFixit logic board replacement guide.

Order a PCB from a PCB manufacturer using the Gerber zip from the Files section.

Make sure to use the following settings:

• 0.8mm thickness
• 4 layers
• A stackup which has 0.1mm between layers 1+2 and layers 3+4. (I used JLC2313, but they have changed their stackup. Looks like JLC04081H-3313 should be compatible.) If your PCB manufacturer doesn't have a compatible stackup, you may need to adjust the widths of all the differential pairs to keep the track impedances correct.

Using the BOM in the files section as a guide, purchase all the components for the PCB. Many of the connectors used in the iPad are no longer manufactured, so you'll need to search around to find them.

You will also need to acquire an iPad A1219 (1st generation iPad, Wi-Fi version. I think the 3G version should also work, but I haven't tested it.)

For each side of the PCB:

• Secure the PCB to a flat surface (it should be heat-proof if you'll reflow the solder while the board is still attached to this surface)
• Gather the components that will be placed on this side of the board
• Using the appropriate stencil, apply solder paste to the board.
• Place the components onto the board, being careful of orientation on diodes, ICs, etc.
• Using a hot air station (or your favorite reflow heating system), reflow the solder.
• Inspect the solder joints with a magnifying glass or microscope, looking for bridges or poorly connected pins. Touch these up as necessary with flux and a soldering iron.

Flip the board over and repeat for the other side.

Clean any flux off the board with isopropyl alcohol.

Before you begin, you may want to unscrew the metal clips from the front screen. If you assemble with these clips in place, you'll need to repeat the treacherous process from step 1 to remove the screen again.

• Attach the CM4 to the board. The CM4's processor should be closer to the center of the board.
• Place the board into the back case of the iPad
• Connect all the cables in the back case:
  • Speakers
  • Headphones/mic
  • Buttons
  • 30-pin dock connector board
• Bringing the screen and the back case near each other, connect the three cables from the screen:
  • Flex cable for the touchscreen
  • LVDS cable for the LCD
  • Flex cable for the ambient light sensor
• Put the screen into the case.

• Put the CM4 into USB boot mode by holding the Volume Down button while turning it on with the unlock button.*
• Follow Jeff Geerling's guide for how to flash an OS onto the eMMC with usbboot.

* Revision 0.0.1 of the PCB doesn't connect the Volume Down button to nRPIBOOT, so this will not work on this version of the PCB. Bodge a connection from the volume down button to TP20.