From KB 955392 there’s lot of info there, but the quickest “one time” install method is to follow “Procedure 1: Basic slipstream steps”
- Install the .NET Framework 3.5.1 feature
- Download the SQL SP1 service pack
- Expand the service pack by using 7zip or the command SQLServer2008SP1-KB968369-x64-ENU.exe /x:C:\SP1
- Install the SQL Setup Support files from the SP1 download C:\SP1\x64\setup\1033\sqlsupport.msi
- Now run the SQL Setup.exe from DVD or network and point setup to where SP1 is Setup.exe /PCUSource=C:\SP1
Note the long term best method is “Procedure 2: create a merged drop” but the above is great for the 1 or 2 installs.