I have a video that I want to trim and rescale, and also slow down by a factor of 2. I am able to do this separately, but I can't find the way to combine all the effects. What am I missing?
It's hard to tell what you're asking without some pictures, but I think you're adding all the effects strips to the original footage. This doesn't work when you're trying to combine more than one effect on the same section of footage. See the following example:
In this (contrived) example, I have a Gamma Cross, a Transform, and a Glow effect that I want to all be applied to that transition between the two strips. If you select one of the purple strips and add each effect to it, then the only effect that will actually be applied is whatever effect is on top.
In order to have all the effects combined the way you expect, you would add the Gamma Cross like normal, then select the Gamma Cross and add the Transform to the Gamma Cross strip, then select the Transform and add the Glow to the Transform strip. This will make each effect combine with the others the way you'd expect them to.
In case you want to combine multiple effects you have to think of it as a "stack". An example:
Strip0: Image strip
add a solid color (shift+a) that we will use to fade-in from black:
Strip1: Effect strip > Color (black) Strip0: Image strip
now add a Gamma cross for the fade-in by selecting first the strip0 and then strip1 (if you first select strip1 you have fade-out ;-) ) and shift+a to add Gamma cross:
Strip2: Effect strip > Gamma cross Strip1: Effect strip > Color (black) Strip0: Image strip
now we want to add a zoom but still have our gamma cross effect (fade-in) working). Well select the Gamma cross from Strip2 and do shift+a and add Transform to zoom in.
Strip3: Effect strip > transform (for zoom) Strip2: Effect strip > Gamma cross Strip1: Effect strip > Color (black) Strip0: Image strip
Now you have a zoomed in image with a black fade-in.
So think of it of trying to select always the top-most element. It can happen that you need to grab some strips and change the vertical order (press g and y when strip is selected) to get the desired effect.
Lengthen the size of the trimmed video to double its horizontal size... this will allow for all of the video to play at half the speed.
Place all your transform strips on top of one another ... you may need to take 59 seconds to delete your old work. Only the last strip should have alpha of 1.00 visible. The other strips should have alpha 0.0 not visible. The last effect strip will have the combined effects and it alone is visible.
In the very worst case. Place the video on the timeline, No transforms, Test. Reduce the complication. Next trim and test. Next Only change speed, test. To determine where are steps first fail. You should have a clear list of what succeeds and fails.