google the model number of your mower (its likely written somewhere under the seat if nowhere else) there are generally manuals online that might have a picture to show you where that spring belongs. In general though, pick a place that it functions correctly and you'll be ok. belts on a lawn mower are far from rocket science. Mine is currently rigged with a trampoline spring and fencing wire.
You need enough tension to keep it from slipping while engaged, but enough play that the clutch can get the pulley clear of the belt.