How do I get a ray to cast downwards?

I recommend using the new WorldRoot:Raycast() since Ray.new() is deprecated.

Also to raycast it downwards simply make the direction the UpVector of the part and make it negative.

local raycastResult = workspace:Raycast(part.Position, -part.CFrame.UpVector * 10)

WorldRoot:Raycast() returns a RaycastResult, not a Ray datatype. If you did this

workspace:FindPartOnRay(raycastResult, part)

it will return an error.

So, what is RaycastResult? A RaycastResult is a returned table when the Raycast was a success. This table contains Distance, Instance, Material, Position, and Normal. Instance is the part that the ray intersected. It will return nil if no part was found in the ray.

while true do
    local raycastResult = workspace:Raycast(part.Position, -part.CFrame.UpVector * 10)

    if raycastResult.Instance ~= nil then
        print("Ray found a part!")
    else
        print("Ray hasn't found a part yet.")
    end

    task.wait(1)
end

Also, instead of WorldRoot:FindPartOnRayWithIgnoreList() or WorldRoot:FindPartOnRayWithWhitelist(), you can use the third parameter RaycastParams.new().

RaycastParams returns a table that lets you choose what part/s will be filtered, what filter type will the filtered part/s set to (blacklist or whitelist), whether to ignore water or not, and lets you set its collision group.

local PartA = workspace.PartA
local PartB = workspace.PartB
local PartC = workspace.PartC

local raycastParams = RaycastParams.new()
raycastParams.FilterType = Enum.RaycastFilterType.Blacklist -- this will make the raycast ignore every filtered BaseParts
raycastParams.FilterDescendantsInstances = {PartA, PartB} -- this is a table of parts that will be ignored together with its descendants, meaning only PartC will be considered in the raycast

while true do
    local raycastResult = workspace:Raycast(part.Position, -part.CFrame.UpVector * 10, raycastParams)

    if raycastResult.Instance ~= nil then
        print("Ray found a part!") -- this will print if it intersects at PartC
    else
        print("Ray hasn't found a part yet.") -- this will print if it intersects at PartA or PartB
    end

    task.wait(1)
end