It's very easy, in fact. Because you don't use the stdlib, it requires handling some language items (the equivalent of having to write your own _start routine, for example). You set up vector tables just like normal, and you decorate things to tell the linker where to put them.

The canonical example is https://github.com/rust-embedded/cortex-m-rt if you want to get into the guts of how it works. For a more user-focused overview, check out https://github.com/rust-embedded/cortex-m-quickstart, which is a good starting point from which to drill down.