Help! Compositing

I was wondering if anyone could offer me some help? I am new and green to Final cut.

Basically I have 2 clips I am overlapping. 2 images of the same person just one a younger version and one of the same person in older make up. I wanted to have the clip start young and blend over the length of the clip to the older person. I know how to change the compositing levels, I just dont know how to do it over the length of the clip, so it starts at 0 and ends at 100% and blends evenly over the clip?

is there an easy way to do this other then trying to split the clips 100 times and changing each one manually? But doing that it will be super choppy.

Any help or advise is appreciated! Thank you!

Wow, I answered this already, and my answer is now gone. Hmmm...

Anyway, let me type it out again.

Few ways to do this. I'll cover the easiest.

Highlight the top clip, Control+V to open the Video Animation panel above it. You'll see the lowest section is Compositing Opacity. On either side is a round blue handle. Grab the one on the top left, and drag it to the right as far as you need it.

The other way is to apply a Cross Dissolve transition to the beginning of the clip, and adjust it's duration as long as necessary.

Finally, you could highlight the clip, place the playhead at the first frame, in the Inspector, click the Keyframe (yellow) button on the right of Opacity in the Compositing section. Then change the level to zero. Have to set keyframe first, set parameter value second, must be done in that order. Then place the playhead where you want the clip to be fully visible, create a keyframe, set opacity to 100.