  In this tutorial we introduce the reader to several theoretical methods of
determining the exciton wave functions and the corresponding eigenenergies. The
methods covered are either analytical, semi-analytical, or numeric. We make
explicit all the details associated with the different methods, thus allowing
newcomers to do research on their own, without experiencing a steep learning
curve. The tutorial starts with a variational method and ends with a simple
semi-analytical approach to solve the Bethe-Salpeter equation in
two-dimensional (2D) gapped materials. For the first methods addressed in this
tutorial, we focus on a single layer of hexagonal Boron Nitride (hBN) and of
transition metal dichalcogenide (TMD), as these are exemplary materials in the
field of 2D excitons. For explaining the Bethe- Salpeter method we choose the
biased bilayer graphene, which presents a tunnable band gap. The system has the
right amount of complexity (without being excessive). This allows the
presentation of the solution of the Bethe-Salpeter equation in a context that
can be easily generalized to more complex systems or to apply it to simpler
models.
