Login to Ad Manager. Scroll down to the bottom of the page and click on:

Click Zone Configuration on the next page:

Make sure you're in the right web property. If you're not, just click on the drop-down menu and select the property you wish to work on:

Once you have the right property selected, click the folder icon directly under the name of the zone you want to add the new position to:  

First, give the position a name. Enter a description if needed. Then, select a page position:

Click the Save button:

The position you created will appear in the zone list:

Click the check box next to the position you created to activate it:

To continue editing the position you created, go to that position and click the pencil icon:

Now you can add dimensions and set limits. To begin, go to the drop down for Dimensions and select a size then click the -> to add it. Add all the sizes this position can take:

The dimensions you added will appear here:

To delete a dimension, click the trash can icon next to it:

Dimensions are auto-saved when the arrow icon is clicked. Now you can set Impression Limits or set a limit for the number of time-based bulk ads.