Now that you baked some cookies and you made your icing, let's decorate a cookie.
I remember decorating sugar cookies by just having my frosting in a bowl and slathering it on with a knife pre-cookie-obsession, that's not the technique we're going to use. Get your frosting/icing to a consistency you can work with - refer to this post for more info. Pour it into a bottle, piping bag or baggie.
I prefer using
piping bags with interchangeable tips, but a cheap sandwich baggie is
an option if you don't want to invest much. If you choose the baggie option, I'd recommend reinforcing the corner you're going to snip with a little tape. Snip a very small hole to start off! You can always cut more off later.
Now take your cookie and outline the area you want iced. This initial outline will hold the rest of the icing in so it doesn't run off the cookie. I like to use one consistency of icing for everything because I am lazy and don't want to mess with two of everything. If you chose to use a thicker and thinner consistency, use the thicker icing to do your piping and outlines.
Now you have a choice, do you want to actually see the outline on the cookie or do you want it to look smooth and outline-free? For beginners I recommend letting the outline show and that means letting the cookie sit for a while so that the icing will harden. If you want to be adventurous and don't want the outline visible, fill the cookie in right away.
Fill the cookie in, but don't overfill or the outline may not contain it. If you chose to make two icing consistencies of icing you would use the "flood" icing to fill in the large areas. Flood tends to be runnier and spreads out more, so don't add too much. Filling to the above level is fine, then just take a toothpick and spread the wet icing to the outline so there are no bare spots.
The advantage of letting the outline harden is that you will not ruin it when you flood and potentially nick it with a toothpick or bump the edge with your finger while you're moving cookies around.
Now you have our iced cookie and you can add whatever extra details you'd like. You can add sprinkles, more lines, glitter... the options are endless. Apparently I quit taking pictures at this point though.