I didn’t know how I was going to fill the balloons, but I knew I’d figure it out. I bought everything I thought I’d need and proceeded to get crafty.

Gather:

Balloons
A seltzer water bottle (it was what I had on hand, but something with a wider mouth might be easier)
Scissors
Confetti that can fit into the balloon neck
Streamers (I used mylar and crinkle paper)
Something to push the filler into the balloon. Nothing too skinny. It needs to be almost as big as the neck of the bottle you use. (I used a large metal Slurpee straw)
Someone to blow the balloons or a helium tank if you want them to float
Curling Ribbon to tie on to the end

Now Fill!

Cut the top off of a 1 Liter bottle (2 liter bottles will have too wide of a cup). Leave a few inches up from the neck so that you’ll have room for the filler to sit before you stuff it into the balloon.
You can blow up the balloons before you fill them to help stretch them a bit and make them easier to fill.
Stretch the entire neck of the balloon over the mouth of your fill-cup. If you don’t get the whole neck over, it will be very difficult to fill. It’s like gathering the top of a sock helps you put your foot in the sock.
See how the neck of the balloon is bunched up onto the bottle? If you have it pushed up like this it’s easier than cramming your filling down the skinny part of the balloon’s neck.
Put the streamers in the fill-cup and shove down with your push-tool (the wide metal straw in my case). Don’t try to fit more in than the uninflated balloon can hold. I found it helpful to add the confetti after almost all of the streamers were inside as it helped the straggling bits of streamer get pushed inside.
A small wad of streamers
Add confetti to the fill-cup and push inside.
Full of streamers & stars!
Remove the balloon from your fill-cup and blow it up!
Tie off the balloon and add curling ribbon to the tail. I very lightly curled the ribbon with the scissors because I didn’t want it to be too curly or girly since this was for a dude’s birthday.
One balloon all done!
Added Bonus: the smaller confetti stuck to the sides because of static. Awesome!
Store the balloons in a staging area until you are ready to decorate. I did mine the night before and put them in the bathtub with the shower curtain shut to hold them in. If you use helium, be ready to tie them to something or weight them each individually.

Some Optional things you could do:

This was one small bag of paper crinkle confetti.
Fluff, fluff, fluff it up! Almost all of this fit into about 4 doz balloons with metallic streamers AND stars!
It was a party EXPLOSION!

Make them fit your theme by coordinating the balloon colors, ribbon color, confetti, and streamer fill
You can fill them with candies or other goodies like money or small toys but remember that everything in them will have inevitable spittle on it if you blow them up by mouth, and will end up on the floor, so make sure edibles are all wrapped. You can also use a mechanical hand pump to avoid the spittle if you like 😛
If you don’t want to buy or rent a helium tank (I found small ones in the $30 range), you can hang them from the ceiling in large groups or entirely covering it for a beautiful effect. If you hang them at different heights and use different sizes, it can look like bubbles! I gathered a group of about 8 balloons, wrapped a piece of strong tape around the ribbon ties somewhat close the the base of the balloons and then taped that to the corners of the room.. If you try to just stick the tape flat on top of the ribbon, it’s likely to slip out. Leave the ribbons hanging to make them look like the balloons are floating!

*A word of caution: Don’t wait until the last minute to do these. The work goes quickly, but it’s easy to get light-headed and pass out if you blow them too quickly or too hard! Leave time for you to take some breaks!