Like everything you create, you must first know what you're creating, even in the cause of militaristic humanoid hot dogs with googly eyes. If you're having trouble coming up with units, first it's a good idea to find a theme to stick with, especially for creating a faction. Starting with a theme of "Magical" for example will be much easier to create a unit for than starting without a theme, which simply has too many possibilities. Also think about what role your unit has in combat. Is it a ranged unit? Is it a tough and slow melee tank? Is it a sneaky assassin? These concepts for the unit's usage in combat will help you when deciding on weapons, abilities, clothing, and stats - almost every facet of creating a unit. Though the unit in the image below has nothing on it yet, it's a fresh, blank canvas for your ideas, historical or fantastical.

Now, we move to deciding on weapons and abilities. What weaponry, if any, will this Wobbler carry? At this stage, your unit is still only rudimentary. If you don't have much pinned down, use this to help your conception - look through the weapon or ability catalogues to get inspiration for a unit.
When deciding on a weapon, you should first know what sort of theming and usage the unit should have. In our example unit's case, it's a fairly generic swordsman, carrying two swords - the Squire Sword and the Short Sword, both medieval. Don't forget colours and projectiles - you could change a Fantasy Good weapon into one resembling one from Spooky or Fantasy Evil by changing light yellows and whites to dark blues and blacks, and change the projectiles from divine bolts to bolts of darkness.
And, apply abilities to your unit, if any - you're not obligated to have abilities on a unit, and most units, especially cheap ones, will rely on their equipment to get by. Our swordsman is going to be a skilled warrior, not masterful, but skilled. For this, we give them the Painter Dodge and Knight Charge abilities - they're faster than a peasant Wobbler, but not as skilful as a Shogun or Red Lady Jade.

At first glance, this isn't particularly important for a unit to be good, but it greatly helps. Slapping on some generic armour to your knightly crusader might be adequate, but focusing on your unit's clothing can improve it a lot more than basic crotch cover. Putting clothing onto your unit helps with conception as well as looks - Is my unit lightly armoured or heavily armoured? Are they an evil warlord with spikes on their armour, or an untrained lackey with only basic clothing? If your unit is lightly or heavily armoured, you can reflect that by adjusting its HP and movement speed in the next section. Also remember theming: though you don't need to stick directly to the category of clothing that fits your unit's origin, make sure that it gives off an overall indication of what culture and time period it's from.
In the case of our swordsman, they're agile and fast, so they have samurai armour and a Renaissance helmet (which both work great for light armour). They have a scarf and arm wraps to provide some extra detail and not leave our unit feeling generic. As a lightly armoured combatant, they don't wear armour on the legs and instead have legacy pants, with some metal medieval boots to provide that extra bit of a lightly armoured unit. Though the example doesn't show this, make sure to colour your unit's clothes if necessary. Slightly different shades of metal armour creates a less appealing and polished look.

Although not as fun as playing wobbly dress up, daydreaming about googly-eyed wizards or browsing the different ways your unit can set things on fire, this can get done quite quickly if you know what to do. The first stat and most important one for unit balancing is the HP. Despite starting at 100, you'll usually want to bump this up. 200 is a decent stat that will work for a lot of melee units such as cheapish shield units or elite units with dodging skill. 100 is good for cannon fodder or unarmoured units with amazing dodging or blocking ability. 300 - 500 is good for tanky units like the Knight, while anything above 1000 will work if you're making a boss unit for your faction, as a counterpart to the Landfall boss units like the Mammoth or King. On unit size, this barely matters and 90% of the time won't be changed unless your unit has huge weapons and/or is a boss unit or giant like the Skeleton or Samurai Giant. Unit weight also isn't that important, just increase it depending on weapon size and tankiness. Movement speed is however quite important, with lightly armoured units naturally having higher movement speed, while armour clad warriors move more slowly than normal. Remember that many weapons will also make units move slower due to the size of the weapon. Attack speed and damage both increase damage, but are best used on different units. Attack speed can work great if you are trying to copy the Poacher or make fast rushdown units, and damage can help if your unit's giant sword doesn't hurt enough.
Our example unit has 200 health to represent the light armour, with slightly higher movement speed and attack speed to keep its damage output balanced. Its damage output is normal, considering how its two swords have enough base damage to deal with things.

There are of course a few things left. Make sure to set your unit's voice to a good fit - I often forget this and have old wizards or frail support units letting out Clubber war cries. Take a nice picture to use as an icon and write a description, either humorous or historical. But there's a mammoth in the room - what about cost? This is highly important, unless you don't know what "game balance" is. Take the auto-given cost, to start. You can use your own judgement on the price and adjust it to fit your needs, but you are not the sole authority, unless you are, which is concerning if you're reading this guide to learn something. You should ALWAYS test your unit, using the dedicated button for it underneath the plate thing in the Unit Creator. The game will roughly match your unit's cost, and unless it's at a very cheap or very expensive price, you will usually get a small group of units. It is perfectly fine if your unit loses - first of all, it has a numbers disadvantage, and second of all, there are plenty of counter units and insta-killers. Try it at least 3 times, and any ratio with a mixed number of defeats and wins will work. Watch and use your judgement - if your unit just loses by a tiny amount, that is definitely different to it dying immediately.
As well as using the dedicated test, go into Sandbox mode and test it there, after putting your unit in a faction so you can use it in Sandbox. Try multiple scenarios - a numbers advantage, a numbers disadvantage, an enemy that counters your unit type, and an enemy weak to your unit type. If your unit seems to be good in some scenarios and weak in others, then congratulations, you've made a balanced unit! If it always wins, maybe nerf its performance or increase its cost. If it always loses, even against what should be an easy fight, then maybe decrease its cost or rework it.

You've just made a good unit, thanks for sticking around! Don't feel obligated to like, favourite, award or comment on this guide, but feel free to! I hope this guide teaches you something, or reaffirms your previous thoughts on good unit creation. And the last rule of creating units: Do it because you want to. Don't force yourself to create units because you feel obligated to. If you're tired from working on your historically accurate 30 unit faction, take a break. You're allowed to, unless you're a Landfall employee.