This is a brief summary about a low-density foam modelling, glass fibre and polyester workshop taken at Prototyping Summer-school @ Howest IDC (Belgium).
http://www.howest.be/summerschool2012/

Credits to Carl for the drawings.

Step 1: Measures of the Head

Bend a metal wire around the head to have the measurements. Once you have the top view and the side view, you can draw them on PU foam block.
We use PU foam because it doesn't react with polyester.
Make sure to draw the center-lines on the block too, so you have a reference and you can make it symmetrical.

Step 2: Cut the Foam

Use paper tape as a reference and guide to cut the foam.
Cut as much as possible, grinding causes more dust.
Don't make the edges round yet, it's easier to control the shape with straight lines.

Sideview:
Draw lines on both sides to check if the sides are symmetrical.
Also use paper tape to cut (knive or a saw). Use tape on both sides to have a good guidance.

Step 3: Rounding the Edges

To round up the edges, you can use sanding paper. You can make a sanding block in order to create smooth sides, because it gives you more control about the movements.
Use double sided tape to stick sanding paper on a piece of wood.
Re-draw center lines after rounding up the sides.

Step 4: Prepare for Polyester

Fix the PU form on the table, you can stick the foam in a wooden stick (not round, so it prevents turning).

Tear the glass mat into pieces, about 10x10cm.
We are using fine and standard glass fiber.

Tools:

Polyester resin
Measuring cup
Mixing stick (wooden stick)
Glass-fiber
Standard roll
De-air roll
Balance (quite precise)
Cleaning paper

Prepare the resin:
Mix 300grams of resin with 9ml of hardener (3%). You can also add a color that you like.
Stir well, in 8form movements.
Once it is stirred, the potlife is about 15minutes.

Step 5: Polyester-ing

Paint the PU foam with polyester-resin.
Add glasmat and push on it with a roller. If you use a brush, don't swipe! Only make pushing moves.

Once there's only layer attached, paint everything again with polyester-resin and add another layer.
Add about 3 layers of 250gr and 1 layer of 30gr.
Use the de-air roller between each layer.
The fine (30gr.) layer gives the helmet a smooth finish.

Work fast, lifetime of the resin is limited (about 15min).

Moist the edge (downside) well.

Step 6: Finishing

After about 20minutes you can cut the side edges with a knife.
The polyester should still be plastic.
If it's too hard, you'll have to use other tools to cut it.

If the helmet is hardened (30-40minutes) you can remove the holding stick and turn the helmet upside down.
Then you can cut out the foam by making V-grooves.

As you can see, I did wait too long and I had to use a dremel to cut the sides.