Well first off, thank you for all the header love! You've motivated me to keep on changing my header when the mood strikes. Which seems to be about once a month. Hmmm. And since you asked so nicely for a little tutorial, here it is.
Obviously I use Blogger, so these instructions are specifically for blogs hosted by Blogger. I upload all photos I use on this site to Flickr, and I create all of my headers over on Picnik. You can sign up for a free account on Picnik, and although Flickr offers photo editing options using Picnik, the editing options are much greater if you go straight to Picnik. You can link your Flickr account to your Picnik account, or select images straight from your computer.
To get started, determine the pixel width on your blog. Mine happens to be 950 pixels, but I've edited quite a bit of HTML to get it that size. Go to Layout on your Dashboard, select Edit HTML and scroll down until you see this:
width: 950 px
The number in red is your magic number. You will create a header this size for a custom fit. Exit Edit HTML before moving on. You don't want to mess up your pretty blog.
Next, go to Picnik and select your header picture. Open the picture to edit.
Crop the picture. I prefer the height of my header to be about a third of its width. Resize the picture to the right pixel size by typing your magic pixel number in the width box. The height will adjust accordingly. Edit your picture if desired.
Select Create then Text to add your blog name. There are many choices in font, size and color. You can also add a sticker (square, rectangle, heart, circle, oval, etc.) and put your blog name on top of the sticker. Save the header when you are done. I save my headers to Flickr.
Go to Flickr, open your new header, click on All Sizes, and copy the URL of the original size.
Go back to your blog, Select Layout, then Edit on the header box. Remove the existing image, then paste the URL for your new header. Select the option to have the picture instead of the blog title. Save changes. View blog. Pat yourself on your back. You give good header too.
Here's the header I made as an example:
I used the Cross Process effect and added a border around the picture. My preferred font is May Queen. The header is OK, but I have a feeling I'll come up with something different for January. The month has only just begun after all. And don't the hearts seem rather February?
Regarding borders, if you see a border around your header that you didn't put there, and you want it to go away, go back to Layout on your blog dashboard, select Fonts and Colors, scroll down to Border Color and change the color to white (assuming your blog background is white). The border will magically disappear and you will be happy.
Regarding the heart garland, I've crocheted a few and they're going into my shop. I'm also keeping a few for myself because they make me happy, just like new headers.