when i did mine
i covered the following parts,
alternator,maf,boost solenoid,battery terminal
then i cut a bit of cardboard to cover the fins in the intercooler,no pics of it.
then i powerwashed the whole engine bay,
then sprayed engine gunk cleaner all over the engine and worked it in with a small metal brush and the odd old toothbrush in the hard to reach areas.
after this i power washed the whole bat again to remove all the crap,
then i removed all tin foil and cardboard and started the engine and allowed to tickover for a few mins while i used an old microfibre towel to soak up the puddles that remained,
turned engine off and dried the excess up,
then sprayed aerospace 303 over the complete engine bay.
shut the bonnet and started the car and allowed to run for a further 10mins, or 5mins and a quick scoot round the block,to get the engine to heat up to allow the aerospace to soak into all the plastics.
and this was my bay when the car had a photoshoot last yr
takes around 25 mins to do the engine every 3months now,
hope this helps