This is a really easy process. But first you have to download your MobileCoin desktop wallet and create an account. Once you've got your MobileCoin desktop wallet installed, it's simply a process of sending and receiving money, which you've done before.
Open your Fennec desktop wallet and choose SEND.
Make sure the top line says MOB and choose SELECT FROM ADDRESS.
Here you will need to add your MOB wallet address. Click the + to add a new address. Open your MobileCoin desktop wallet and enter your password to unlock your wallet. Once your wallet is open, you will see your address (it is the square filled with lots of blue, white, and green letters and numbers). Click on the address block to copy it.
Back in your Fennec wallet, paste your address into the address box on the bottom. Name your address something memorable. Click SAVE. Fennec will make your re-enter your password. After you do, you will see a green button pop up that says SAVED SUCCESSFULLY.
Click on the address box. This will open the send screen. Click into the box that says MOB and enter the amount you want to send.
Hit the big orange button that says SEND. You should see another green button pop up that says SEND SUCCESSFULLY! Your MOB should appear in your MobileCoin wallet within a few seconds. If you don't see it appear automatically, log out and log back in to your wallet.