As requested, here are some basic instructions on how to create a custom header for your blog. First things first, you NEED to have some type of scrapbooking software (photoshop, photoshop elements, microsoft digital image standard, SOMETHING!) I used Microsoft Digital Image Standard up until November of last year when my adoring husband brough home PSE for me (Photoshop Elements) You should know that I use a PC, so mac's might be different. I'm not sure.
1. Download theme kit from scrapbooking site of your choice to your computer. It's easier if you make a new folder in your documents folder that is named something like "blogging" or "headers" or "backgrounds".
2. Open your documents folder and find the file you just downloaded. It will most likely have to be unzipped, or the icon of the folder has a folder with a zipper on it. To unzip the file, right click the folder and select "extract all" it will take you through the extract wizard and you will be able to choose where the unzipped files go. Once you are done, you should be able to view the thumbnails.
3. Open your editing software/program. The instruction pictures I am using are in PSE.
Click File, New, blank file and it should take you to that little box where you can choose the dimensions you want. I usually do a 5x7 as a basic size, but you can play around with it. Make sure that the resolution is set to 100. You'll see that highlighted. It will open up a new blank page.
4. go to file, open and select the background you want for your header, you can also just leave it white. I usually use a scrapbook paper. When it opens, click and drag it over to the blank file and then resize it using the corners to fit your header as you like.

Do the same with the elements/embellishments, resize them to fit your header
Add custom text, pictures, etc

Save your file as a JPEG
To add them to your blog, log into your blog and click on CUSTOMIZE in the top right hand corner. Once you get to that page, click on the "LAYOUT" tab. click edit on the top long bar, which is your header.

It will pop up a little window. Make sure that the shrink to fit box is checked and you have selected "instead of title and description" Then select upload from your computer and it will upload.

When you can see the preview, select save, and you are done!!! Then you can go to view blog, and see your beautiful header made by yourself!!
I hope these directions were easy to understand and answered some of your questions. I am by no means a photoshop expert, but those are the simplest directions. You can play around with different styles and things like that!! Enjoy!!