It is really easy and requires only a minimum of effort. And the photos don't have to come from Flickr, you can import them from Facebook as well, copy/paste the exact URL or upload them from your computer - this requires a bit more effort, though! ;-)
Go to the Mosaic Maker page at Big Huge Labs (worth checking out in general, there's lots of fun stuff you can do with your photos).
Play around with the settings: number of columns, rows, border colour etc. Pick where you want your photos will come from and then click the 'Create' button. In this example I have chosen one of my Flickr sets with embroidery patterns, using 5 colums and 6 rows.
Once your mosaic has been created (it may take a few moments), you should see something like this. Above the image are a few options for stuff you can do with the mosaic. Edit it if you are not completely happy with it. Share it on Flickr, Facebook, Twitter or email it. And you can save it, of course. Below the image is a box with the HTML code for the links to each image. If you are using someone else's photos in your mosaic, as I do in the Mosaic Monday ones, which are made from my Flickr favourites, you should include these links.
And that is pretty much it. :-)