Here are the steps:
1) Offline: make a new wallet with seed and everything.
2) Offline: copy the master public key
3) Online: make a watch-only wallet using this master public key
I think you have already done steps 1-3
4) Online: go to send tab, fill in the boxes, click "preview" and copy the unsigned tx
5) Offline: transfer the tx from last step to your offline wallet, import it and click "Sign"
6) Online: transfer the signed tx from last step to your online wallet, import it and click "Broadcast"