How do you use ray for a placing system?

If your model has a Primary Part, you can take that and cast a ray from it, and then check if the part it hits is parented to the players base.

--Sample Code

local playerBase
local modelChecked
local maxDistance = 128 --Or some other number, I don't know.
--Assign all of this

local ray = Ray.new(modelChecked.PrimaryPart.CFrame.p, Vector3.new(0, -1, 0) * maxDistance)

local part, pos = workspace:FindPartOnRay(ray, modelChecked)

if (part.Parent == playerBase) then
    --Do whatever you want to do in here
end