For higher quality assistance, could you please link to or upload the tutorial here? Also, some screenshots of your interface at the points you're having trouble would be helpful.
Regarding step 23, something is wrong either in the instructions or your understanding of them. You cannot have two selections active at the same time, so if you've control-clicked a layer, any commands that would create another selection will either replace that selection or be unavailable. My suspicion is that you need to simply click the target layer, then do the color range selection.
For step 8, I don't know what it is you're trying to do, but I can explain the flow feature. Flow and opacity are related, but they function differently. In short, flow determines how much paint you're putting on the canvas, and opacity determines the actual opacity of that paint. In technical terms, flow controls the opacity on a per-sample basis (applied before the stroke is made) and opacity controls it on a per-stroke basis (applied after the stroke is made). The best way to really understand it is to just play with both features on a blank canvas and see how they differ. Also adjust the spacing control in the brush palette, as that can make the difference clearer.
In any case, if you have flow at 10%, you won't get sharp definition in your lines—even at their strongest, they'll fade out at the beginning and end of the paint stroke.
There's not enough information in your other questions to take a stab at answering them, unfortunately.