Measuring your bed for custom sheets is probably the most critical of measurements for bedding products.  A fitted bottom sheet must fit a mattress in a certain way.  The corners must be at the corners of the mattress (not off the side, or somewhere else along the edge), and the elastic around the bottom must be tucked slightly under the mattress itself.   It takes a very carefully arranged set of numbers to bring that about, and you are the only one who can provide it for us.

The first step in getting a custom sheet that fits, is to measure the length and width of your mattress.  There is no legal requirement for mattress manufacturers to conform to "standard" sizes in making their beds.  It could be a mistake to assume they stuck to standard sizes on your bed.
Next is the tricky part.

The depth shouldn't be measured from cord-to-cord. As you can see from this picture, the bottom-most cord is above the box spring, and the top cord is below the upper loft of the mattress.  As tempting as it may be to use the cord as a neat measuring point, you'll end up with sheets that just don't fit.

Instead, place the end of the tape measure on the top of the box spring, as shown here.  Then place your hand on the top of the mattress and measure up to it.  As you can see here, the depth of this mattress is 14".

Remember to round off your depth to the nearest inch.

Also, bear in mind that the sheets you order will actually be one inch deeper than the mattress measurement you give us.  This is to provide enough sheet to tuck under the depth, not just fit to the bottom edge of your mattress.

While these measurements will certainly be used to make the fitted sheet fit properly, they are also carried over to the top sheet. 

The width of our top sheets can be found by adding the width of the mattress, twice the depth, and another ten inches.  (IE: A 78x80x13 mattress gets a top sheet that is 78+13+13+10 inches wide, or 114") 

The length of our top sheets follows a similar formula, incorporating the length of the mattress, the depth, and a further 8 inches. (IE: A 78x80x12 mattress gets a sheet that is 80+13+8 inches long, or 101")

Of course, if these dimensions aren't to your liking, you are more than welcome to request a specific set of top sheet finished dimensions.  Keep in mind that you should calculate about 2% shrinkage into your custom finished size if it's in 100% cotton (including flannel).