Hi,
Usually the easiest way is to just use a particular post/page for the front page:
https://codex.wordpress.org/Creating_a_Static_Front_Page
On that page, use a mashup shortcode, for example:
[mashup query="all" width="100%" height="500px"]
If you want to actually modify the theme files, you can use the same shortcode but call it from PHP with the do_shortcode function. There are some examples in the documentation that show how to do that.