I assume you've read http://electrum.org/offline_wallets.html
Sometimes I need to read another description of something to get it to work. Here's how I do it (in the simplest sense):
Using your watch-only wallet, when you're in the Send tab, you'll see 'create unsigned transaction.' Fill out the form and hit that, save the file to something.
Reboot... Pull out your Ethernet cable (I reboot with Tails) and load up your offline wallet. From Tools, Load Transaction. Sign it, save it.
Reboot...
Load up your watch-only wallet and do a Load Transaction with the file you saved and then a Broadcast.