To start creating a new project with Slide, select the New option from the toolbar or the File menu. A new blank project window will be opened, in which you can start creating your model.
There are two things you will generally need to do before you start entering your model boundaries:
Select Limits from the View menu, to set the initial limits of your drawing region.
Select Project Settings from the Analysis menu or the toolbar, so that you can configure the main analysis parameters (direction of failure, unit system, method of pore pressure calculation, etc).
See the Quick Start Tutorial for a lesson on how to create a simple model with Slide.
TIP: when the Slide Modeler is first started, you will always be presented with a new blank project window named "Slide1". You can immediately start creating a model in this window. If you are not starting a new project, then simply Open the file you want to work with (the new blank project window will be automatically closed).