readme: update

README.md

@@ -4,4 +4,46 @@ You can use the overlay provided by this flake along with your own IDA installer
 
 ## How to Use
 
-WIP.
+First, whether flakes are used or not, the IDA Pro installer must be made available for Nix.
+
+This can be done by checking it into a private Git repository, supplying the build the path to the package, fetching the installer via the network, etc.
+
+For example, the installer can be added to the store like so:
+
+```sh
+nix store add-file ida-pro_90_x64linux.run
+```
+
+This will then give you the store path, which is later used to instantiate the package.
+
+### With Flakes
+
+In your flake's inputs, import the overlay:
+
+```nix
+ida-pro-overlay = {
+    url = "path:/home/msanft/dev/ida-pro-overlay";
+    inputs.nixpkgs.follows = "nixpkgs";
+};
+```
+
+Then, apply it to your `nixpkgs` instance with [`nixpkgs.overlays`]()
+
+```nix
+nixpkgs.overlays = [
+    ida-pro-overlay.overlays.default
+];
+```
+
+Then, you can instantiate the package, supplying it with your locally available installer:
+
+```nix
+environment.systemPackages = [
+    (callPackage ida-pro {
+        # Alternatively, fetch the installer through `fetchurl`, use a local path, etc.
+        runfile = /nix/store/z83flk6c9fm9li3gs13vbamq2szg9rwf-ida-pro_90_x64linux.run;
+    })
+];
+```
+
+IDA Pro should then be available in your system environment.