This might be a totally unnecessary step-by-step but I felt like it wasn't real intuitive when I first got a Flickr account so I thought I'd make it easier on our new Flickr users who are participating in the 21 Day Challenge.
Sign in to your Flickr account then click the "Upload" link at the top of the page.
Click "Choose photos and videos." This should bring up the standard upload dialog box. Select which pictures you want to upload from your computer.
The file names will show up in a box, like this one. You can add more or click the "Upload Photos and Videos" button.
A little bar will show your upload progress then Flickr will ask you very politely if you would like to add a description. And you do.
Add your outfit info and click "save."
This will take you to your Photostream. Click on the photo you just uploaded.
Then click "Actions" above your photo and it will give you this drop down menu. Select "Add to a group"
Select "21 Day Challenge" from your group list and you're done!