How do I repair this friction ABS kitchen sink pipe fitting
The white fitting should be on top of a small compression ring, so when reassembling the pipes you would add the white female threaded fitting, the compression ring (thin side pointed down), and then the black male threaded side of the fitting. When you tighten this by hand, the compression ring will get squeezed between the two pipes and make a water tight seal.

For the slope, 1/4" per foot of run should be your target.