This is an imageless tutorial.
For those that already do digital scrapbooking, this will be easy to follow. Your going to create a header using some of the same basic steps as you would a normal scrapbook page from scratch.
You can also use brag book and scrap page templates to create the header. You'll have to adjust the size.
A typical blog header is 800 pixels wide and either 200 or 300 pixels high. The header on my Totally Templates blog is 800x300 and the one on the Freebie Vault is 800x200.
Open a new image in the dimensions you wish your header to be.
Now import items you wish to be a part of your header (use the File>Open As Layer option in the image box). Don't worry if the items are too large for your header.
To resize the layers: Right now I recommend clicking on the eye in your layers dialog box and hiding each layer you are not currently working with (click on the eye to unhide). Adjust your background image. For each layer you want to resize, you need to click on it in the layers dialog box (making it your active layer) and then returning to the image box and select !! IMPORTANT !! Layer>Scale Layer. If your whole image scales, you selected the wrong scaling method. Image>Scale Image resizes every layer. Also, remember to scale it down in increments. You cannot scale down then back up because you will lose quality. If this happens you can also undo or open another copy as layer again.
It doesn't hurt to save this file every once in a while. Make sure your extension is xcf so you retain your layers.
Now, adjust all of your items one by one to your taste. You can scale down, adjust color, add brush work, add text, etc.
If you are using a script-fu technique on one of the layers, I highly suggest you click on that layer, Edit>Copy>Edit>Paste As New, then add the script on the new image, then Edit>Copy, go back to header and Edit>Paste (click on single blank page in layers dialog box to add this image to it's own layer), arrow down to place into position. Delete your old layer if needed. If you didn't export this image first, most script-fus dump the other layers permanently, and you would have lost all of your work so far. Standard filters do not do this, but with 2.4 you have to be careful to watch which ones will.
When you are happy with your work, save as an xcf file first, in case you want to alter it down the road such as adding a snowman or fall colors for the seasons. Once you save, merge layers and save as a jpg or png.
WAIT!! Do you want blinkies or tags for siggies? Scale down your header to use for forums. Sizes are based on forum rules or you can use others as an example. You can also rework your header to make a matching tag for your package previews. This is another example of why saving in layers is important.
TIP: Once you create a matching logo from your header, you can turn this into a full color brush. Do not change it to grayscale or it won't be in color! In 2.2, go to Script Fu>Selection>To Brush. In 2.4, Edit>Copy as Brush. You can always save your text portion as a brush as well. It will come in handy. If you want the brush to pick up colors in the active palette, then go to Image>Mode>Grayscale, then save as a brush based on the method for your version. I have another post here that goes into more details on creating brushes.