I am no expert at creating headers at all, but I would like to share with you how I make my headers if you are thinking of beginning a new blog and have no clue how to make a header.
If you don't make headers regularly you won't want to spend money on expensive software to help you make headers so I use paint. Paint comes free on all windows computers, there are other types of software available for mac that do the same thing. Using paint does take a long time to get it exactly how you want with the tools it provides but the good thing is it's very simple to use and doesn't take a long time to work out, unlike other software's like Fireworks, which may be confusing and expensive for first time users.
If you are starting your blog up on Blogger, which I recommend you do as it is extremely easy to use and works perfectly, then you will need to set the size of your page to exactly 951x295 for it to fit, i spent a while trying to get this right but there's nothing worst than completing your header and realising it doesn't fit.
This is what it should look like, you may need to zoom out to be able to shrink the page, the zoom bit I have cropped out to fit but it is in the corner.
Once you have this shape you are free to add what you want to your header. The only real big fault with paint is that once text is written and you have clicked off the text box you cannot move the text box if you need to later on, this is why I find the select tool (next to the paste button in the top left) very useful as you just highlight over your text and you can move it.
Paint is very simple to use and does not create complex headers, but if you are a new time blogger I'd really recommend this as it's free and the tools are easy to use.
YouTube headers as I found are considerably harder to make, as you usually find that the writing is not centered in the middle and gets cropped out. It took me a while to get my writing centered into the middle so it didn't crop out. For YouTube I used a canvas of 2048x1152. It took me a while, mainly through the process of trial and error to keep adjusting the writing to get it centered and not cut off. Try your best to centre the writing in the middle of your canvas then upload it to your YouTube header and check if it is cropped off, if it is use the 'select' tool to drag over the writing and move along a bit. It shouldn't take long until you have it in the right spot.
Thanks for reading! Hope this could help you.
Any more questions you can contacts me at the bottom of the page.