New 3.5 Hair System

This setup was made for fast hair generation with the new hair system introduced in Blender 3.5

The blend file comes with 3 guide collections, in each you can add a mesh (or curve) that will act as the hair generator guide for a geonodes hair system.
for meshes just add an edge crease to the edge you need as the root of the hair.

Shape Keys

This setup gives a very precise grooming with very realistic details added on top.
I've included a shape key to the guides for frontal and back positions and a deactivated cloth simulation that can be used for posing

Shader

the principled BSDF shader works fantastic with this setup without major modifications.

take a look at the hair guides for this hairstyle below:

Setup

WHAT ARE HAIR GUIDES?

simple planar meshes that give shape to the hair style, the only requisite they have is they need to have the root edge loop marked as Mean Crease in edit mode.

take a look at the hair guides for this hairstyle below:

HOW TO INCREASE/ DECREASE THE NUMBER OF PARENT HAIRS?

you can edit your Guide Mesh by adding more edge loops (edit mode: CTRL+R) or removing edge loops (edit mode: Select edge loop to delete and hit X > disolve edges)

however to keep the Guide Mesh manageable you can also multiply the number of parents via the Hair Object's geometry nodes using the option Double Guides:

HOW TO CONVERT TO OLD PARTICLE SYSTEM FOR EXPORT?

instead of trying to convert the the entire hair object with thousands of children you may want to convert only the parent hairs to use as strands in the old particle system, to which you can use the old children method of your choice, to do so you need to select the Hair Object and delete the children and other shape modifiers:

after that make sure that your Hair Object is also tied to your Mannequin or custom model or scalp model by goind to the Hair Object Hair Data Tab, this will act as the emitter of your new particle system:



and lastly go to the modifier tab of the selected hair object and click the arrow icon next to the camera icon of the "Hair From Mesh and Curves" modifier and  click apply, this makes it so that the Hair Object is no longer relying on the geometry nodes, and now can be converted to the old particle system by going to Object>Convert>Particle System

your custom model or scalp model should now have the corresponding hair system, note that the resulting system may be too dense for practical application of hair dynamics or game engines, if that's the case consider modifying the Guide Mesh and Geonodes Subdivisions before converting.