I start by creating some guides so I know that I will be getting the hole in the middle of the shape. I click and hold at the side of the screen on the page rulers. I then drag guides out and position them roughly where i want them. I double click on the guide and a dialog box opens that allows me to edit the value so i can define exactly where they are. I enter the value to the nearest 10. I then reposition the rectangle so it snaps to the guides.
(You can edit your guide properties by going to File:Document Properties and adjust the snap sensitivity)
I create a 3mm circle which will be the hole in the Pendant. I drag another guide onto the screen. This guide is to centre the hole. I know the circle is 3mm so I set the dimensions for the guide 1.5mm off the center of the rectangle. This way when i position the circle on the guide it will be in the middle of the rectangle.
I drag another guide which I set 2mm down from the top of the rectangle and then reposition the circle so it is on the guides.
Zoom out and group the circle and the rectangle.