How do I go about using FindFirstChild() in a different script for an instance?

I am not very good at scripting but this is what I would do:

local brick = Instance.new("Part")
brick.Name = "Brick"
brick.Parent = workspace

if game.Workspace:WaitForChild("Brick") then
   --Do what you want here
end

If this helped at all please accept this answer.

Try this

-- Finds it
local a = Instance.new("Part")
a.Name = "A Part"
a.Parent = game.Workspace

if game.Workspace:FindFirstChild(a) then
   --Lol
end

or this

-- Waits for it
local a = Instance.new("Part")
a.Name = "A Part"
a.Parent = game.Workspace
local b = game.Workspace:WaitForChild(a.Name)

if b then
   --Lol
end