As suggested by scarletsquig
In 10 easy steps!
1) Firstoff, have a base idea of what you want. Kind of a no-brainer, but it really can help you figure out what might be a better choice for filling certain unit slots.
2) Come up with unit names and what sizes you want them to be available in. Keep in mind you MUST have at least 1 unit that can be at least a Regiment/Section, as H/M, Armor, War Machines, and Aircraft require one to be used. Honestly, the more the merrier here as figuring out the point cost of a Section/Regiment is very easy if you already know the value for a Troop/Team
3) Figure out an appropriate statline (Sans Special Rules). Keep in mind that Flying/Jump Troops automatically changes Speed to 10 in 1st edition. Don't forget also that there are no official army lists that have a 2+ to hit (By to-hit I mean Melee/Ranged for KoW and Hit for WP), nothing in KoW that has a 7+ or better Def (At least as far as I know...), and at most you should only have around 2 shots/melee attacks per figure in the unit (ie a Team shouldn't have 25 shots or 25 melee attacks).
4) The points. It helps to find a model from an official rulelist that has as close of a statline as you can, and modify from there. I haven't made an army list in a while, so my memory may be a bit fuzzy here, but this should be approximately right (These are all assuming you're figuring it out for a Troop/Team or single other model. DON'T figure out a section/regiment or platoon/horde without figuring out the cost for a Troop/team unit first, even if the troop/team is not part of your final army list):
(Note: These all assume you're giving it better stats then the comparative unit. If it's worse, simply reduce it by that many points instead)
Speed (Average: 5): +10 points if there is significantly more movement (Fairly arbitrary, something like 3 -> 6 or 7 for example. Ignore this for units with Fly/Jump Pack)
Hit/ Melee (Average 4+): +10 points for every point of difference (ie, 4+ -> 3+ is +10 pts, 4+ -> 5+ is -10 pts)
Ranged (Average 5+): Same as for Hit/Melee
Def (Average 4+): Same as for Hit/Melee
Attacks/Firepower (Average 1/model in unit): +5 pts for every 5 additional attacks (Keep in mind all KoW and WP pts are in multiples of 5, so when in doubt, round up)
Range (Average 24): +10 pts for every additional 12 in range
Nerve (Average 10/12): +10 pts for every additional 1/1 (Nerve can be simplified to a single number, and then adding that number to 8/10 to find the nerve stat. This is important for calculating the Nerve of a unit of a larger size like a Troop -> Regiment)
Special: Can buy X special weapons in warpath does not cost additional points.
5) Special Abilities: The addition of all special abilities is +10 points, except for the exceptions mentioned below. In the case of a Special Rule with (X), it counts as +10 pts per X:
Breath Weapon/Flamer: +10 pts per 2X (ie, Flamer (4) would be +20 Pts)
Fly/Jump Pack: +20 Pts
Craven/Yellow-Bellied: -10 pts.
Blast: +10 pts per 1d3, +20 pts per 1d6 (So Blast (2d6) would be +40 pts)
Lumbering/Shambling: No cost
Transport: +10 pts per 5X (ie Transport (20) would be +40 pts)
Regeneration/Bloodbath: +20 pts
6) Army Special Ability: Every army gets an additional Special Ability that usually affects all their units. This is worth 10 pts as mentioned in step (5), and is not calculated into the cost of each additional unit (You can choose an ability or abilities worth more than 10, but then you have to start factoring that in to unit cost).
7) For Sections/Regiments and Platoons/Hordes (Note that this is different for KoW and WP, so don't mix them up!):
Troop/Half Regiment -> Regiment: Points = (2 * Troop) - 10. Nerve = Troop Nerve * 2 (See step (4) about figuring out your single Nerve stat)
Regiment -> Horde: Points = (Regiment * 2) -10. Attacks = Attacks * 2 (Including doubling any Breath Weapon/Flamer/Zap! special ability). Nerve = Regiment Nerve *2
Team -> Section: Points= (Team * 2) -10. Attacks = Team Attacks * 2. Firepower = Team Firepower * 2. Nerve = Team Nerve * 2.
Section -> Platoon: Points= (Section * 2) -10. Attacks = Section Attacks * 2. Firepower = Section Firepower * 2. Nerve = Section Nerve * 2.
8) Available unit upgrades: For WP, these are usually BFGs or special melee weapons. Usually, the number a unit gets is 1 for Teams, 2 for Sections, and 4 for troops.
For KoW, Banners cost 15 points and Musicians cost 10 points.
For other unit upgrade ideas, I'd advise looking at existing army lists. Usually, if you'd be reducing a stat by an equal number of points that the upgrade would raise it by, make it a free optional upgrade. Otherwise charge a number of points equal to the difference in cost between the lost stats and the upgrade.
9) BFGs and Special Weapons in Warpath: For a BFG, again find a similar existing BFG and apply steps (4) and (5) to find the new cost. For melee weapons, do the same but count each extra die rolled as +10 pts (Instead of +10 per 5 as per the guidelines in step (4)).
Do this for everything, keeping in mind most:
KoW armies have 6-7 infantry units, 1-2 Cavalry units, 1 Large Infantry unit, 1 Large Cavalry unit, 2-3 War Machines, and 7-8 Heroes/Monsters
WP armies have 6-7 infantry units, 2-3 Ordnance units, 3-4 Armor units, 1 Aircraft unit, 3-4 Heroe/Monster units, 4-5 BFGs, and 2-3 Special Melee Weapons.
10)And you're done!
Please keep in mind this is a very rough outline, and will need playtesting in order to make sure it is costed appropriately. But be sure to have fun with it, and please let me know of any comments, feedback, or criticisms in the comments!