First, open or create a \custom\ folder in the same location where you found or created the \levels\ folder. That means they should be both in the same parent folder. You will need to make a subfolder in the \custom\ folder named the same name as the subfolder you created in the \levels\ folder.
Then, create a text file (in subfolder you created in the \custom\ folder), and name it any single character that can be used in filenames, like Q or W or some weird thing like | or ~ that may or may not work. This is the character that you will place in a custom level to spawn the contents of this text file.
The first line defines what in game object you want to spawn. You can use any ingame object, but the most useful ones I will provide in a legend later on in this guide.
The following lines define the modified stats. They consist of a string of letters and a string of numbers. These can be anywhere on the line and even mixed within each other, but it will always resolve to one string and one integer number. Note that if the parser can't find a valid string it will assume that it reached the end of time file and won't try to parse any further lines, so make sure you spell all the strings correctly.
Here's the example character provided, "Q", which makes a charcoal superscamp:
hp = 10
Note that the ='s are optional and the numbers and letters can be mixed. I would not recommend this because it ruins readability, but it's up to you to maintain consistent syntax.
Here's the legend I promised of useful objects, but there are other objects you can spawn if you want.
clan - clannfear
obs_rand - uses chance as the chance to spawn
obs_rand_invert - uses chance as the chance to not spawn
foe_rand - uses chance as the chance to spawn
rug_spawner - this is a random bottle
s_potion - speed potion
Here's a key of the valid strings to put in text files:
"hp": The amount of hp an enemy, potion, or crate has. 1 hp = 1 fireball
"chance": The chance it has to create an item, these will have 'rand' in their name.
"looking": Which degree angle the enemy starts looking in. 0 degrees is to the right.
"bspd": How fast an enemy is. Clannfear are 8 and scamps are 5.
"lookchange": How fast they look around. Default is 1.
"viewrange": How much peripheral vision they have. Clannfear have 45 and scamps have 55.
"hearing": Their hearing radius. Clannfear have 100 and scamps have 200.
"dam": How much contact damage they deal. Clannfear have 20 and scamps have 5.
"scorer": How many points they are worth. Both are worth 5.
"imageblend": The color to blend the thing. Works on everything, but it wants hex input while the parser only takes numbers. 0 is black. It does work, use a hex converter to find the number in decimal.
"imagealpha": The opacity of something. 0 is completely transparent and 1 is opaque. Due to "."'s being flushed out this might as well be a invisible/not Boolean.
"firetime": Scamps have a cooldown after each shot determined by this number, the default is 20.
"isundead":A Boolean for if the enemy counts for Meridia's quest.
"frostres":Multiplier of frost damage. ".5" is half "1" is full damage.
"fireres":Multiplier of fire damage. ".5" is half "1" is full damage.
"stuckins":Boolean of whether arrows can get stuck in an enemy.