Here is an example of keyframing, to create a Dissolve envelope where the object dissolves in and out, you could start by creating the following simple envelope. Choose any Flying Window FX and then click on the object you want to dissolve, then drag the slider in the envelope window to 10% time and drag the Dissolve slider to 1% value.
A new dissolve envelope, created by dragging the dissolve slider.
After creating the initial envelope, click on Copy then move the slider to 30 for the Paste value. Click Paste. This will copy the keys from 10% and paste it at 30%, to create the following:
The envelope after copying the keys from 10% to 30%.
Now move the slider back to 0% and click on Copy then move the slider to 20% for the Paste value. Click Paste. This will copy the keys from 0% and paste it at 20%, to create the following:
The envelope keys ready to Copy & Paste.
Now do this several times to create a loop:
The final envelope after several alternating Copy and Paste operations.