create new background..example-red

step 2
Make 2 stroke paths with pen tools
-1 is white -1 is yellow

step 3
Go to Object > Blend > Blend Options, enter Specified Steps = 20.

Then with the 2 strokes selected, go to Object > Blend > Make.

This will blend 2 white strokes and create an abstract wave effect. If you want to change the Blend step, you can go to Object > Blend > Blend Options or double click the Blend Tool(B) on the tool bar.

here’s the result..:-)