Read chapter 4 in the manual to start.
You sacrifice a great deal of the flexibility of a modeler when you rely on a guitar amplifier for your volume. Regular guitar amps are designed to color the sound coming into their preamp and out of their speaker, and the modeler is designed to do that too, so your resulting sound is doubly colored, meaning that the amp in the preset you choose will not sound as it’s supposed to.
If your amp has an effects loop then insert the modeler into the loop’s return, following the directions on p.31.
If you want to only use the modeler to supply effects without amp modeling, follow p.34, plug into the preamp-in (the regular guitar input) and don’t add an amp or cab block to the preset in the modeler.
Typically we’d use a neutral power-amp into a neutral sounding speaker cabinet and rely 100% on the modeler to create the sound we want, as shown on p.28, but YMMV.
I have some really nice boutique tube combos, and love them, but don’t bother trying to use them with my Fractal units. If I need to I’ll take my pedal board and run everything as a traditional board and amp but usually I run straight into the amp. When I use my Fractal modeler I take two FRFR powered monitors and run into those and rely on the modeler to give me the sound I want.