NUMERICAL MODELING REQUIRES THAT YOU DISCRETIZE BOTH TIME AND SPACE

This can be a daunting task when you do not have experience. The best way to learn is to dive right in and build a grid, then learn from your mistakes. Of course this is a task that is likely to be like writer's block, so use the following guidelines to help you along the way.

If your grid has serious problems in your class project, we may explore and discuss the issue, rather than rebuild the grid to a "perfect" condition because the goal in class is not to end up with a top;notch model, but rather to learn from the problems that arise. The time available for a term project negates the possibility of creating models without substantial shortcomings.

First consider the plan view of your grid. For a finite difference grid, lines between cells need to be orthogonal and extend the entire width of the grid. Consequently, any detail defined in the interior of the grid is extended all the way to the edges. Most finite difference codes allow the width of cells along rows to vary.
Considerations for laying out the plan view of your model grid.

Now consider the cross sectional view of your grid. MODFLOW allows you to vary the thickness of layers on a cell by cell basis, but each layer must extend across the entire model. Consequently, if geologic units pinch-out, you must deal with this by changing the properties of the layer.
Considerations for defining the layers of your model grid.

Finally consider how you will discretize time.
Considerations for defining the time steps of your model.

Smaller cell sizes and time steps provide more accuracy, but one wonders how small is small enough?