how do I pass a variable from server to client with a remote event?

I assume that you have a RemoteEvent in ReplicatedStorage. First, you have the fire the RemoteEvent with the player argument, it refers to the client you want to fire.

game.Players.PlayerAdded:Connect(function(plr)
    game.ReplicatedStorage.RemoteEvent:FireClient(plr)
end)

Then, add arguments after plr to pass the arguments to your LocalScript.

game.Players.PlayerAdded:Connect(function(plr)
    game.ReplicatedStorage.RemoteEvent:FireClient(plr, game.Workspace.Part)
end)

In the LocalScript, write this.

game.ReplicatedStorage.RemoteEvent.OnClientEvent:Connect(function(part)
    --Write what you want to do on the client side.
end)

game.ReplicatedStorage.RemoteEvent:FireClient(player, part) --Add more commas and values to pass more things.
--Client
game.ReplicatedStorage.RemoteEvent.OnClientEvent:Connect(function(part) --Just add it by the order
    part.Transparency = 1 --For example
end)