This is widely documented throughout the internet already, yet it still took me hours to get sorted so I thought I would share a few tips.
In my case I could not add .Net 3.5 from Add/Remote features within Windows itself, this failed.
The next step was to try using Dism.exe commands which also failed (This does work, I will explain why it did not work for me)
With my Installation I installed Windows 8 and then proceeded to run the upgrade to 8.1 from the Windows Store.
The installation of .Net 3.5 for Windows 8 & 8.1 are DIFFERENT which is not widely documented.
If you upgrade to 8.1 then you need to enable this feature with Dism.exe from a Windows 8.1 disc, your version 8 disc will not work.
Here is the code to do this: Dism.exe /online /enable-feature /featurename:NetFX3 /All /Source:E:\sources\sxs /LimitAccess
This is where E:\ is your CD Drive.
This code is the same for Windows 8 but you need to use a Windows 8 disc not 8.1
Alternatively I will post links to both the Windows 8 & Windows 8.1 .Net standalone installer, I used these to get .Net installed correctly.
Windows8 32/64bit .Net 3.5 Standalone: http://bit.ly/1h9lSi9
Windows8.1 32/64bit .Net 3.5 Standalone:http://bit.ly/1bEiBto
Thanks to My Digital life forum for the 8.1 Link (http://bit.ly/18VzL3i)