To start off, you'll want to select a smaller region of your video file to use as reference, since testing on the entire thing would take forever. The recommended way of doing this is by using awsmfunc's SelectRangeEvery:

import awsmfunc as awf
out = awf.SelectRangeEvery(clip, every=15000, length=250, offset=[1000, 5000])

Here, the first number is the offset between sections, the second one is the length of each section, and the offset array is the offset from start and end.

You'll want to use a decently long clip (a couple thousand frames usually) that includes both dark, bright, static, and action scenes, however, these should be roughly as equally distributed as they are in the entire video.

When testing settings, you should always use 2-pass encoding, as many settings will substantially change the bitrate CRF gets you. For the final encode, both are fine, although CRF is faster.

To find out what setting is best, compare them all to each other and the source. You can do so by interleaving them either individually or by a folder via awsmfunc. You'll usually also want to label them, so you actually know which clip you're looking at:

# Load the files before this
src = awf.FrameInfo(src, "Source")
test1 = awf.FrameInfo(test1, "Test 1")
test2 = awf.FrameInfo(test2, "Test 2")
out = core.std.Interleave([src, test1, test2])

# You can also place them all in the same folder and do
src = awf.FrameInfo(src, "Source")
folder = "/path/to/settings_folder"
out = awf.InterleaveDir(src, folder, PrintInfo=True, first=extract, repeat=True)

If you are using vspreview or VSEdit, you can use output nodes to set each clip to a different node. Then you can switch between them using the number keys on your keyboard.

# Load the files before this
src = awf.FrameInfo(src, "Source")
test1 = awf.FrameInfo(test1, "Test 1")
test2 = awf.FrameInfo(test2, "Test 2")
src.set_output(0)
test1.set_output(1)
test2.set_output(2)

If you're using yuuno, you can use the following iPython magic to get the preview to switch between two source by hovering over the preview screen:

%vspreview --diff
clip_A = core.ffms2.Source("settings/crf/17.0")
clip_A.set_output()
clip_B = core.ffms2.Source("settings/crf/17.5")
clip_B.set_output(1)

Usually, you'll want to test for the bitrate first. Just encode at a couple different CRFs and compare them to the source to find the highest CRF value that is indistinguishable from the source. Now, round the value, preferably down, and switch to 2-pass. For standard testing, test qcomp (intervals of 0.05), aq-modes with aq-strengths in large internals (e.. for one aq-mode do tests with aq-strengths ranging from 0.6 to 1.0 in intervals of 0.2), aq-strength (intervals of 0.05), merange (32, 48, and 64), psy-rd (intervals of 0.05), ipratio/pbratio (intervals of 0.05 with distance of 0.10 maintained), and then deblock (intervals of 1). If you think mbtree could help (i.. you're encoding animation), redo this process with mbtree turned on. You probably won't want to change the order much, but it's certainly possible to do so.

For x265, the order should be qcomp, aq-mode, aq-strength, psy-rd, psy-rdoq, ipratio and pbratio, and then deblock.

If you want that little extra efficiency, you can redo the tests again with smaller intervals surrounding the areas around which value you ended up deciding on for each setting. It's recommended to do this after you've already done one test of each setting, as they do all have a slight effect on each other.

Once you're done testing the settings with 2-pass, switch back to CRF and repeat the process of finding the highest transparent CRF value.