You will need to know how to put together a home pc from scratch. If you can't, stop now and learn. From there it's minor stuff, software front ends, power supplies, hardware options (like GPS, Wi-Fi, etc...), finding out how you want it to look once it's in. The last part I am sure can do wth no problems.
If you have neither the time or patience to learn, you may consider bringing in someone to put the unit together and configure all the software, then you can work your magic to make it look OEM.
Either way there is a LOT of information on this site. The search funtion is your friend