1) Download the plugin. The best places for these are the Extend section (they are usually all 1.8+) or the older MyBB mods section (this is where the 1.6 generally are).
2) You should go ahead and open the zip file and locate the plugin php file. These are normally found by following this path:
3) Open the plugin with Notepad++ (I recommend) and find the "compatibility" section. This is found at the top of the file so should be visible upon opening.
If it says 16 that means that you need to change it to 18 in order to work with MyBB 1.8+. This is the only thing you need to do to get old plugins to work with 1.8.
2) You should go ahead and open the zip file and locate the plugin php file. These are normally found by following this path:
3) Open the plugin with Notepad++ (I recommend) and find the "compatibility" section. This is found at the top of the file so should be visible upon opening.
If it says 16 that means that you need to change it to 18 in order to work with MyBB 1.8+. This is the only thing you need to do to get old plugins to work with 1.8.