Creating a new, playable class from scratch sounds like a difficult task. It is, to an extent. Be prepared to spent hours into trial and error.. especially when it comes to a skill mechanic (affixes, triggerable, charge bar system..), particle or UI. But fret not, you can always ask away in the Torchmodders forum if you have further question regarding class creation.
Note that for new modders, you can opt to clone skills from vanilla classes and use it for your own classes. Some opt to copy skills from other custom classes. Hopefully you're going to at least ask the original author(s) for their permission and give them full credit for their work. It's a common courtesy as a human being and as a modder, respecting the work of other modders.
If you are planning on publishing your class mod to public, I advised you to at least try to create some new, original skills for your own class. Grab the Magma Spear skill from Embermage. Tinker around with the skill affixes or cooldown. Edit the description to fit your class theme, change the particle to fire a lightning bolt instead of fire. Be creative.
Originality is a good aspect to promote your class mod, or any mods in general.
But keep in mind that the GUTS editor has quite a few engine limitations and unfinished feature that would limit some creative modding ideas.
In order to create a playable custom class, you'll need these folder/files. I'm going to give a general explanation on what does these folders do. You should find them inside the MEDIA folder in your TL2 directory, extracted by GUTS during the first time you run the editor.
As an example, navigate to the following folders and take a look at the .DAT files inside. Use program like Notepad++ to open them.
1) UNITS/PLAYERS/WANDERER - Basically the character itself. There is base file, m_file (male character) and f_file (female_character). They are linked with UNITTYPES, MODELS, SKILLS and SKILLTREEOVERRIDES. And WANDERER refers to Outlander class.
2) UNITTYPES (outlander.dat,outlander_male.dat, outlander_female.dat) - Mostly used to determine what kind of gears can the class wears. You can make your class be able to wear Berserker and Engineer gears at the same time.
3) SKILLS/WANDERER and AFFIXES - These two are usually related to each other. Some skill doesn't actually uses affixes.
4) SKILLTREEOVERRIDES - The skill UI of the class. Determines the placement of every skill in the UI.
5) UNITUNIQUEMENUS and PIECES - Related to the charge bar of the class. The former determines how the chargebar works while the latter determines the chargebar's tooltip description.
6) SPAWNCLASSES (questreward_mainline.dat, questreward_randomevent.dat.. etc) - Determines the quest rewards for each classes. Without this your custom class may not getting any rewards during quest completion.
That's it, you've now figure out most of the important folders in the class creation. You can start cloning the above folders/files and rename them accordingly to fit your class idea.
However if you're willing to put more effort, time and have creative artistic skills.. you can take a look into these folders/files as they could truly define your class's originality.
1) PARTICLES - All things related to skill particles.
2) MODELS - Meshes and animation file for certain skills.
3) MISSILES - Related to skills that fires a projectile (e.g Glaive Throw)
4) UNITS/ITEMS - Weapons, armor for the class.
5) UI/ICON - Varying from skill icons, class portraits (in the character creation), charge bar textures..
6) UNITTHEMES - Can be used to determine a skill theme/particle, status effect, or even certain skill functionality.
Important note : Get a new GUID for every skills that you have copied. This is to ensure compatibility between your copied skills and the original skills.