First, determine if you are dimensioning the model to indicate size and proportion or, if you want to drive changes to the model with dimensions.
Both scenarios are described below. Changes to a model are done with tools that use dimensions to define the size of a change. Usually dimensions appear on screen when using a tool to make an edit. For example, when using the Pull tool to change the height of a cylinder, a dimension appears that shows the size of the change:
To create a dimension that measures the height of the cylinder, click the Ruler icon in the Options panel or on the mini-toolbar. Click the bottom of the cylinder to create a temporary dimension that measures the height. Creating dimensions for reference is done with DesignSpark Mechanical Annotation tools. Use the Dimension tool on the RS/Allied tab to create dimensions that measure length, angle, and other characteristics.