Hi g.taylor.3,
I hope all is going well, thank you for participating on the Forum. In the Fusion 360 electronics library editor open an existing library or open a new library editor session.

With the library open, click on the grid command and set your grid to mm.

With the grid selected. Use the SMD command to add a pad. Before placing the SMD pad, make sure you select the correct size from the drop box, or double click the size field to add one.

If you need an arbitrary shape, draw a polygon on top of the SMD pad. Make sure to duplicate the polygon and change the copy to the corresponding Stop layer.
Let me know if this helps.
Please 'accept as solution' if my reply resolves your issue.
Edwin Robledo
Tech Marketing Manager