You need to deselect the 'Smd' tool or you'll keep adding new pads every time you click somewhere.
Go back to the left-hand toolbar, and click an action like 'move' (highlighted in yellow below) that doesn't create something new:
The best way to precisely position your pads is to manually enter their exact X + Y position (relative to center/home/zero) in the properties dialogue box.
You can also tweek the length and width of the pad and a bunch of other important settings here, but we'll cover some of that later.
Now that you've deselected the Smd tool (by selecting 'move' or something similar), move your mouse over your pad, right-click on it and select 'Properties' from the popup menu:
Which brings us to our next logical question ... "How do we know exactly where to position our pads?!?"
Going back to the datasheet we mentioned earlier, we can see that there are four 0.6 x 0.6 mm pads in our 2.0 x 1.6 mm rectangular footprint.
This is where you always need to do a bit of math, and calculate positions and sizes based on the data present in the datasheet.
Manufacturers will rarely display every possible value in the datasheet, but there is almost always enough information to infer the numbers you need with a bit of simple division, addition and subtraction.
The first pad that we'll place is the one in the top left-hand corner, which is marked as 'pin 1' in the datasheet:
Since we know that pads in eagle as placed relative their center point, it's easy to calculate the exact 'X' position for this pad:
- Find the distance from the center of our part (the zero position indicated by the dash-dot lines) to the right-hand edge of our pad (0.5mm)
- Calculate the distance from the right-hand edge of our pad to it's center position, which is the pad width / 2 (0.6mm / 2 = 0.3mm)
- Add them together (0.5mm space + 0.3mm to the center of our pad), and we know that our pad should be centered at -0.8mm on the X axis.
- Now enter '-0.8' in the first textbox beside 'Position' in our properties dialogue:
Similarly, we can calculate the center Y position for the pad by taking the 0.35mm from the zero position to the bottom edge of our pad, plus 1/2 the pad height, meaning 0.35mm space + 0.3mm to pad center, = 0.65mm on the Y axis.
Enter this value in the second box beside 'Position:' to set the Y location: