First you have to create Pharrell in-game, using Create A Player, then you open plyrface.viv in the SGSM/players folder using EA Graphics Editor. Extract JR Smith's file (should be named jrsmith) then using FshX open JR Smith's file and then 'Import' your Pharrell CF. Save the file and re-name it to phwilli and put it in your SGSM folder. You have to get a headshape for your player so just open up plyrlodb.viv (same place as the plyrface.viv file) and find JR Smith's name. Extract it, rename it to your players name (in this case Pharrell Williams, you would name it phwilli) and put it in your SGSM folder.
Hope that helped.
Btw, All the programs (such as FshX, EA Graphics Editor) can be found in the downloads section.