Crude way of making plated slots in Eagle
As far as we know Eagle doesn't offer an option for making plated slots. Here are two hacks we found on the subject.
Plated slot using holes
Make a pad for bottom and top layer.
Tip: When you're done with the top pad just copy it, and change its layer to the bottom layer.
Using the hole command put overlapping holes beside each other depending on your slot dimensions.
The gerber should look like this. Some wedges may remain using this method because of the areas uncovered by the holes.
Plated slot using the milling layer #46
Slots need to be made to the PCB manufacturer's specifications. If no information is provided, try contacting them about it. Here's Seeed Studio's tip on how to send them PCB file with slots.
After doing the above top and bottom pads, draw a slot outline in 46 Milling layer.
Note that the slot doesn't show up in the gerber.
Add some text specifying there is a plated slot there. This way, you make sure they are aware of what you want for this slot.
This method returns a bunch of errors when running the DRC. Make sure you don't miss other errors because of it.