Considering Loading in analysis and design
In the general case, it's an iterative process, where you design all of the stages in isolation first, then adjust the design of each stage to account for the actual values found in the previouis pass. Usually, this will converge quickly.

However, if you can design your stages up front so that the input impedance is an order of magnitude or two (i.e., 10&times; to 100&times;) greater than the output impedance of the previous stage, then no iteration should be required.

Alternatively, you can plan ahead what the interface impedances are going to be and design each stage for that specific impedance on each end. Usually, you have enough degrees of freedom to make this a straightforward task.