Mesa Express 5:50 Plus Head to Boss ME 80 connection?

I am new to amps with effects loop and new with the Boss ME 80 Multiple Effects .
How do I connect the ME 80 to my Express 5:50 Plus Head.

The 5:50 Plus owner manual says, on page 14,

To use the EFFECTS LOOP;

1) Connect the SEND jack to your processor’s Input
2) Connect the RETURN jack to your processor’s Output
3) Adjust the processor’s Input/Output Level attenuator to achieve unity gain while plugging the RETURN cable in and out making fine adjustments at the processor until you here no level difference."

The rear Panel of the ME 80 has:

- Gt. AMP. OUTPUT R & L/MONO

Also, do I connect the guitar in the Mesa Head or at the back of the ME 80?

Any help would be appreciated

As I understand it, the ME80 is designed to be used with a clean guitar amp. It is supposed to be connected as:

Guitar -> ME80 input
ME80 OUT (Mono) -> Guitar amp input

This will put all the effects, including the time-based effects, in front of the preamp gain, so not the best approach. This will work if you are OK not using the lead channel of the 5:50, or at least with minimal gain/overdrive. You would have to rely on the ME80 distortion/OD effects.

If the ME80 can stand line-level inputs, then you could ignore the ME80 overdrive/compressor effects and run

Guitar -> 5:50 input
5:50 FX Send -> ME80 Input
ME80 Mono out -> 5:50 FX return