1) In the Library tab, click on "Add a new game" then select "Add a Non-Steam Game". A window with a list of random programs should show up.
2) Click on browse and search for your javaw.exe file.
- -For 32-bit and 64-bit users: "C:\Program Files\Java\jre#*\bin\javaw.exe"
-For 64-bit users who uses 32-bit Java: "C:\Program Files (x86)\Java\jre#*\bin\javaw.exe"
* # is your Java version. If you are using Java 6, replace # with a "6". If you are using Java 7, replace # with a "7".
3) Right click on "javaw" in your Steam library and click on "Properties". A window should show up.
4) In the Target text box, type in "-jar" without the quotation marks. Then type in the path to your Minecraft launcher with quotation marks around it.
5) Click on close and launch the game on Steam. You should see the usual Steam Overlay pop-up after you log into Minecraft. Press your shortcut for the Steam Overlay. You should get something like this:
Are my instructions too confusing? Here's a video tutorial made by WACOMalt