docs: Make emscripten path warning more obvious.

docs/Emscripten.md

@@ -4,7 +4,7 @@ Building in Emscripten - asm.js/WebAssembly - works on Linux and WSL.
 
 You will need to install the emscripten compiler. See https://emscripten.org/docs/getting_started/downloads.html for complete instructions, but generally you should just browse to your desired directory and:
 
-```
+```shell
 git clone https://github.com/emscripten-core/emsdk.git
 cd emsdk
 ./emsdk install latest
@@ -14,16 +14,21 @@ source ./emsdk_env.sh
 
 You should now have commands like `emcc` in your path, and an `EMSDK` environment variable set ready to build your project.
 
-In your project directory:
+In your project directory create a new build directory, enter it and configure your project:
 
-``` shell
+```shell
 mkdir build.em
 cd build.em
 emcmake cmake .. -D32BLIT_DIR="/path/to/32blit/repo"
+```
+
+:warning: Make sure to include the `-D32BLIT_DIR="/path/to/32blit/repo"` parameter to the `cmake` command (even when building the SDK examples). You must specify an absolute path here when building with Emscripten.
+
+Once cmake has finished configuring your project you can build it:
+
+```shell
 make
 python3 -m http.server
 ```
 
-Make sure to include the `-D32BLIT_DIR="/path/to/32blit/repo"` parameter to the `cmake` command (even when building the SDK examples). You must specify an absolute path here when building with Emscripten.
-
 Finally, open the URL given by Python's HTTP server in your browser and open your project's .html file.