Right I will give you a list of what you need and the what to do.
A. Hobby knife
B. various grades of wet & dry sand paper
C. Putty , milliput , squadron putty or aves apposite sculpt
D. Surface primer , grey and or a sandy beige colour
E. Brushes I actually use sable but synthetic are ok too
a flat brush for large areas like the whole head , round brush for eyebrows and what not and a 00000 brush for eyes.
F. Your paint. , tubed acrylics or pot acrylics
G. A sealer , in UK I find humbrol matte the best as testors dullcote can't be bought anymore in uk apparently the mr hobby stuff is very good.
These are aerosol versions , but the brush on Vallejo stuff is very good.
H. Small things like mixing palette super glue and 2 part apposite
So here is a list if what you need to do
A. You head could be a flawless cast which is very rare , but if you find seem lines and small holes the can be tackled very easily.
So remove the seem lines with the BACK of the blade on the craft knife.
Then smooth with wet & dry and only ever use it wet 400 grit should be fine.
Then either use squadron green or white putty for very shallow or pin holes.
If something like the nose tip is missing use aves apoxie sculpt ( amazing stuff) or milliput mix the 2 parts together fill the hole the best you can.
Let it dry over night and then shape with wet & dry.
B. wash your head with a tooth brush and washing up liquid , rinse all the soap off and let dry on its own.
Do not use towels just naturally dry on the top of a radiator.
Then get your primer and spray the head with a few light Coates and let dry for at least 24 hours frustrating I know.
Also I may add to handle the head the least you can with your bare hands so use powder free latex gloves also put the head on a stick and hold the stick.
C. Yet your paints ready and start mixing them the way you find easiest.
If your using tubed acrylic paints then on a mixing palette either mix your paints together using an old brush and then thin down with acrylic medium as water tends to dilute the colour or you can use dionised water that they use for car battery's.
I would say thin like skimmed milk , and apply with the large flat brush with even brush strokes and do not over work the area.
Let it dry or use a hair dryer to speed things up.
Also make sure your brush is wet before painting.
And take your time and do not rush and you will need to add upto 4/5 coats of paint to get a good even coverage.
Let it all dry and seal your work with laquer.
Then add shading and highlights sealing your work after you add paint each time.
Do not try and speed up the laquer drying with a hair dryer as it will go satin.
Hope the all helps just paint and try to enjoy it.
Good luck
Jay