Larry covered pretty much everything already. I have a 1st gen Saffire, which also uses MixControl and a couple other thoughts came to mind.
First, a couple more references. Since I tend to misplace hard-copy manuals, the pdf for the 6i6 is here. The video I usually give to folks is from Graham at TheRecordingRevolution.com and is here (a little shorter, and mix focused).
In MixControl, a couple additional things:
Inst vs. Line (pre-amp inputs 1&2), Hi Gain vs. Lo Gain (Line inputs 3&4) - Using Inputs 3&4 on the back of the Scarlett (used for Line inputs), be sure to set the input levels appropriately in the MixControl Router section. For the cassette deck, you will probably want these set to "Hi Gain" which is the default. I am not familiar with the SC-8850, and that may be "Lo Gain." If using the pre-amps (Inputs 1&2 on the front), be sure to set the Router inputs to "Line" if using either of these, and "Inst" for the mic. As Larry mentioned, be sure all gains are set to zero on all equipment before making/breaking connections or powering on equipment.
In MixControl, you can also stereo link the even/odd inputs (the interleave/infinity symbol below the faders in MixControl). When working with Inputs 3/4 from a unit, this will give a stereo output. They are hard-panned left and right by default respectively. On some older cassettes, the right track can be lower volume, so leaving them split and recording them as mono channels into CbB, leveling them, then bouncing to a stereo track may be better. You may also find it easier to record the cassette into CbB first (as a separate step to recording instruments/vocals), then using the faders inside CbB to control playback levels via the "DAW output" sent to the Scarlett.
On my unit, actually opening MixControl makes routing into a DAW smoother (even if MixControl is not "actively" used). I typically power on my unit, open MixControl, verify it "locks" on the MixControl GUI, then open a DAW.
Again, I have not used the Scarlett, but the Input settings (#1 above) are in the Router section of my MixControl at the bottom.