How exactly do you make methods for parts (like :Destroy())?

I came up with an idea to make a ModuleScript that calls a method that finds the Humanoid of a player and kills it...

(BTW apologies for the lengthy question)

I've been trying to test this idea in a regular script that doesn't kill nor is a modulescript. Here's the script I've got.

function getData(self) -- function with "self"; a.k.a. le method that I am hoping to get to function
	if self.Parent == nil then -- if part has already been destroyed
		warn("Part has no parent") -- warn (I'm too lazy to use error() lol)
	end
	print("Getting data on part...") -- this is just for fun
	wait(1)
	print("BEEP BOOP")
	wait(0.3)
	print("The selected part's name is "..tostring(self.Name)) -- Print the Name of the part
	wait(0.4)
	print("The selected part's FullName is "..tostring(self:GetFullName())) -- get the full name
	wait(0.4)
	print("The part's BrickColor is "..tostring(self.BrickColor)) -- get le brickcolor
	wait(0.4)
	print("The part's RGB color is "..tostring(self.Color)) -- get RGB
	wait(0.4)
	print("The part's Reflectance is "..tostring(self.Reflectance))
	wait(0.4)
	print("The part's Transparency is "..tostring(self.Transparency))
	wait(0.4)
	print("And finally, the part's position is... *que drumroll*")
	wait(1)
	print("..."..tostring(self.Position).."!!!!!!!1") -- finally le position
end
wait(1) -- countdown
print("3")
wait(1)
print("2")
wait(1)
print("1")
wait(1)
game.Workspace.i_am_a_part:getData() -- attempt to call the function as a method on the workspace part

But I get from the output:

>> getData is not a valid member of Part

When what I expect is:

>>3
>>2
>>1
>>Getting data on part...
>>BEEP BOOP
>>The selected part's name is i_am_a_part
>>The selected part's Full Name is game.Workspace.i_am_a_part
>>The part's BrickColor is Medium stone grey
>>The part's RGB color is 163, 162, 165
>>The part's Reflectance is 0.7
>>The part's Transparency is 0.3
>>And finally, the part's position is... *que drumroll*
>>...-6, 0.5, 4!!!!!!!1

However... If I run this in the same script instead:

game.Workspace.i_am_a_part:Destroy() -- destroy the part
wait()
local partCheck = game.Workspace:FindFirstChild("i_am_a_part")
if partCheck then
	print("Part not destroyed")
else
	print("Part destroyed.")
end

I immediately get the output: Part destroyed.

------------CONTINUED-------------

I've kept this question in my browser un-posted for a long time, hoping I could make :getData() function without posting this, but so far I have not succeeded in doing so, so I guess I'll just post it...

Here is the script at the point I posted this:

local Game = {} -- table to imitate "game"
local mt = {
	__index = function(self, i) -- make sure Instances exist in the table when called
		self[i] = i -- Attempt to set the value of the object to the Instance
		print(self[i]) -- meant to print the name of the instance
		print(self[i].ClassName) -- meant to print the ClassName of the instance
		-- iT = i.."ch" (These two lines were me thinking of spreading the metatable's code to the table... rejected it though. Wasn't gonna work anyways)
		-- iT = {}
		return self[i] -- return the instance
	end
}
setmetatable(Game, mt) -- set the metatable of Game to mt
function getData(self) -- ye olde method attempt
	if self.Parent == nil then
		warn("Part has no parent")
	end
	print("Getting data on part...")
	wait(1)
	print("BEEP BOOP")
	wait(0.3)
	print("The selected part's name is "..tostring(self.Name))
	wait(0.4)
	print("The selected part's FullName is "..tostring(self:GetFullName()))
	wait(0.4)
	print("The part's BrickColor is "..tostring(self.BrickColor))
	wait(0.4)
	print("The part's RGB color is "..tostring(self.Color))
	wait(0.4)
	print("The part's Reflectance is "..tostring(self.Reflectance))
	wait(0.4)
	print("The part's Transparency is "..tostring(self.Transparency))
	wait(0.4)
	print("And finally, the part's position is... *que drumroll")
	wait(1)
	print("..."..tostring(self.Position).."!!!!!!!1")
end
wait(1)
print("3")
wait(1)
print("2")
wait(1)
print("1")
wait(1)
Game.Workspace.i_am_a_part:getData() -- will it work? o wait, i'm cursed, it'll never work

In this current state, when run, the output prints Workspace when the last line runs - we want that. But then after that, it prints nil. I presume this means that it's calling the table object... but not the instance? Like, weird. Also, then it errors Workspace.Script:44: attempt to index field 'i_am_a_part' (a nil value). I do see why though.

This is the part that confuzzles me though: If I change self[i] = i to self[i] = game.i, it simply errors >>i is not a valid member of DataModel. Soo... I guess that's a good stopping point.

(Note: This is my first question, so I apologise for any inconveniences.)

RESPONSE TO LONG-AGO FIRST ANSWER: I don't see why I can't just finish the script and keep going on and implement it as a method (pun simultaneously intended and unintended) to kill players easily; I've already made progress that clearly can be progressed on. Also, I barely even can understand the code given (to clarify, the best game I've created is basically just an average baseplate, poor decoration, smolboi-level scripting, and several free models (generally of guests)). I mean... i just wanna... finish my script...

2:52 - Script, line 98: sweetkid01 has died

You can't create a custom function for roblox instance. You can however make a so-called "proxy" also known as object wrapping or sandboxing.

Basically, you make a fake userdata and set its metamethods to make it do the same things as the real instance, with the exception that you add the stuff you want.

Here's a simple wrapper example:

function Sandbox(obj)
	local fakeobj = newproxy(true)
	local meta = getmetatable(fakeobj)
	meta.__metatable = getmetatable(game)
	local customs = {
		getData = function(self)
			--do stuff
		end,
	}
	meta.__index = function(s,n)
		if customs[n] then
			return customs[n]
		else
			local s,e = pcall(function()
				return obj[n]
			end)
			if not s and e then
				return error("Sandbox error: " .. e)
			end
			if type(obj[n]) == "function" then
				return function(s, ...)
					return obj[n](obj, ...)
				end
			else
				return obj[n]
			end
		end
	end
	meta.__newindex = function(s,i,v)
		obj[i] = v
	end
	meta.__tostring = function()
		return tostring(obj)
	end
	return fakeobj
end

Regarding to my comment:

local part = game.Workspace.i_am_a_part
local partclone = part:Clone()

part:Destroy() -- destroy the part

wait()

local partCheck = partclone
if partCheck then
    print("Part not destroyed")
else
    print("Part destroyed.")
end