To make the Guile integration useful, you have to design some kind of hook into your application that application users can use to cause their Scheme code to be evaluated.
Technically, this is straightforward; you just have to decide on a mechanism that is appropriate for your application. Think of Emacs, for example: when you type ESC :, you get a prompt where you can type in any Elisp code, which Emacs will then evaluate. Or, again like Emacs, you could provide a mechanism (such as an init file) to allow Scheme code to be associated with a particular key sequence, and evaluate the code when that key sequence is entered.
In either case, once you have the Scheme code that you want to evaluate,
as a null terminated string, you can tell Guile to evaluate it by
calling the scm_c_eval_string function.