This tutorial was written for the Flash Designer software, which allows you to create flash animations in a much easier way than by using Macromedia Flash. You may get Flash Designer here.

Launch Flash Designer and create new document. Choose "Frame" > "Frame Size" and enter 200 x 300.

Draw background rectangle. Choose "Rectangle Tool" and draw a rectangle. To fit it to the entire frame choose "Edit" > "Align" > "Fit to Frame".

Choose "Item" > "Line and Fill" > "Line Width" > "None".

Set gradient fill. Choose "Item" > "Line and Fill" > "Gradient Fill". Choose "Horizontal" style. Set left and right colors. Click OK.

Draw a circle. Choose "Circle" tool, hold Ctrl key and draw a circle outside right edge of the document frame. Choose "Item" > "Line and Fill" > "Line Width" > "None".

Set gradient for the circle. Choose "Item" > "Line and Fill" > "Gradient Fill". Choose "Horizontal" style. Set left and right colors opposite to background rectangle. Click OK.

Animate the circle. Choose "Item" > "Intro Animation". Check "Enable", choose "Fly" and set duration to 3 seconds. Click OK.

Loop the animation. Choose "Item" & "Sprite Animation", the animation will loop infinitely.

Set frame stop to stop frame looping. Choose "Frame" > "Frame Delay" and check "Stop". Click OK.

Hit F9 to preview Flash file. The circle should fly from left to right in infinite loop.

Duplicate the circle. Select it and choose "Edit" & "Duplicate". Use "count" 5 or more. Click OK. Resize circles in a random way:

Randomize fly over times for each circle. Select each circle, choose "Item" > "Intro" > "Intro Animation" and change duration time (between 2.5 to 4 seconds).

Hit F9 to preview Flash file. You should get the final effect.