Choose a row egg and clean it's surface with water and a little bit of soap if necessary.

Empty, clean and dry it.
You can follow this tutorial: http://www.cutoutandkeep.net/projects/blow-out-eggs

Please, clean it well! You will have to put something inside it (and you don't want it to smell).

Decore it as you want.
At the beginning I tried to paint it using watercolors but it didn't work because the egg is waterproof. You will need other kind of paint. You can add glitter, stickers or whatever you want.

(Tutorials on Easter eggs can inspire you).

Put your message (or a gift) inside! Moreover, I put inside paper stars, color sequins and glitter. Yon can put candies or whatever you want.

Paper stars how to: http://www.cutoutandkeep.net/projects/origami-stars

Cover up the holes (remember you have two).

I covered the little one (the one I blew though) with white glue and the biggest one with a sticker.

(I will do a pokeball egg, I need it!)

Think of how you are going to present it.

I personally think half of the success of a gift (and nearly anything in this life) is vested in the way you show it, so work hard on that point.

You are finished, so you can give it to your friend/mate/whatever and enjoy!

Advice: ask him to crack the egg against something high, so you can see how glitter and so on fall. It can be really amazing.