hêhh vì cái vụ này của bro mà /me mất công search mấy chục trang trên GOOGLE và phải email để có cách làm đó ((hhihi nhưng cái này hay phết) --> đằng nào cũng phục vụ mình và anh em có nhu cầu xài.
Không chắc có vừa ý bro chưa nhưng theo /me nghĩ là khá OK.
1. To start off, you will need to make the snow.
Even before that, the background must be adjusted. Create a new slide in a new or existing presentation, and choose Format | Background. This brings up the Format Background dialog box that you can see in the screenshot below.
Click on the dropdown menu and select a dark background, preferably black. If a black swatch is not available, click the More Colors options and choose a suitable color. Click Apply to get back to the slide.
2. With the dark background ready, it's now time to create the snow. In PowerPoint's Drawing toolbar, select the oval AutoShape.
Tip: If the Drawing toolbar is not visible, choose View | Toolbars | Drawing.
3. Hold down the Shift key and start drawing so that you end up with a perfect circle rather than an oval. Resize the circle as required, while making sure the circle is not too big (see screenshot).
4. Double-click the circle to summon the Format AutoShape dialog box (see screenshot below). In the Colors and Lines tab, under the Fill section, click on the Color dropdown menu and select Fill Effects.
5. This will bring up the Fill Effects dialog box - make sure that you have selected the Gradient tab. Under Colors section, select One color and change Color 1 to White.
Under Shading Styles section, select From center. This will change the white circle into a white-gray gradient circle as seen in the sample picture.
6. But this is not what we want. We will need to remove the gray portion so that the gradient fades from white to transparent. To achieve that, adjust the "To:" under Transparency section as 100%. This will make the gray completely transparent (see screenshot to match settings). Click OK to get back to your slide.
7. Now, move over to the Line section in the Format AutoShape dialog box (see screenshot). Under Line Color dropdown menu, select No Line. Click OK and your first drop of snow is ready!
8. Next, select the snow and click Ctrl + D to duplicate it - repeat this 20 or more times to create more instances of snow. Drag all the snow off to above the slide area and distribute them evenly as shown in the figure below. You do not need to align them horizontally.
9. Now is the time to start with the animations. Select all the snow instances and choose Slide Show | Custom Animation. This will activate the Custom Animation task pane.
Click Add Effect | Exit | Fly Out. If the Fly Out option is not available, click the More Effects option to bring up the Add Exit Effect dialog box - you'll find Fly Out in the Basic category.
Ensure that the first effect has "Start:" as On Click, and the rest of the effects have "Start:" as With Previous. Also check that the Fly Out direction for all the snow instances is set to "To Bottom".
10. Click on the first effect in the Custom Animation task pane. Hold down Shift key and scroll down to select the last effect. This will allow you to highlight/ select all the effects. Right click on the effects and select Timing to bring up the Fly Out dialog box (see screenshot)
Set the Repeat to Until End of Slide. Click OK.
11. You can now play around with the speed. Set the first effect to Very Slow, second to 6 Seconds, third to 7 seconds, fourth to 8 seconds and fifth to 9 seconds. Repeat the speed settings to rest of the effects. By doing so, the snow will not be falling at the same speed. This will also make the effect more realistic.