I know that some people have trouble with this so I will tell you how I do it, probably not perfect (I am no expert!) but it works for me. I use Adobe Photoshop but I have included my instructions for Picasa as well.

Taking the Photograph

I prop my layout up against an object such as a pile of pizza boxes or a bucket of pencils so that the layout is relatively upright.
Take a photograph using lots of natural light and no flash. Make sure you get a bit of ‘background’ around it; this can be cropped out later and don’t worry if it isn’t perfectly straight you can edit that in Adobe Photoshop or Picasa (free editing program from Google, just Google Picasa to find it).
Download the image to your computer.

Editing in Photoshop

Open the photograph in Photoshop
Click the crop button and set the restrictions in the top left of the screen to 1000px by 1000px (72 resolution is fine for blogging)

Crop the photograph so that all the image is within the square with a little excess background around the layout (not too much)

Right click on the background layer in the layers palette and choose ‘Create Layer from Background’

Choose Edit | Transform | Distort and using the little square handles that appear around the image to drag the corners of the layout into the corners of your image window. Press enter to commit to the changes.

Choose Edit | Transform | Warp if some of the sides of your layout look curved and the image window is still not full of your layout. Just drag the little circular handles to get the layout looking just right and then click enter/return to commit to the changes (ensure the image stays straight, you can make it look wonky if you are not careful!).

Go to Image | Resize and resize the layout to 600px square.
Go to Filter | Sharpen to sharpen your image
Choose File | Save for Web & Devices. I usually choose the ‘High’ or ‘Medium’ quality options here depending on the file size that you can see under the right hand image (if you have clicked the ‘2 up’ tab along the top). I like to keep mine around 100kb but check the right hand image to see what the quality will look like.

Editing in Picasa

Open Picasa and double click the photograph in your library to open it in the Picasa editing section
Use the crop button and set the drop down list to ‘Square: CD Cover’
Crop the layout as close to the edges as possible making sure to get it all in. Click the Apply button.

Click the straighten button and drag the slider to get the image straight
Right click on the image and choose ‘save’
If required crop the image again to fill the frame and straighten if necessary
Click the ‘I’m Feeling Lucky' button to correct the colour and brightness of your image
Go to the ‘tuning’ tab at the top of the screen and adjust the sliders if necessary

Go to the ‘Effects’ tab and click the ‘Sharpen’ option, click Apply.
Now save your image by clicking the ‘Export’ button on the toolbar at the bottom of the screen and choosing your desired options.