You can crochet one square, doubling back on the top and bottom, but an easier way is to crochet twelve smaller squares and then whip stitch them together. Make your afghan three squares wide and four squares long. Size your squares according to the desired finished dimensions of your afghan.
A standard queen-size blanket is 90 x 100 inches (adjust for amount of overhand desired). Determine how many stiches per inch, then multiply by desired dimensions and add 1.
Using a medium weight yarn, chain 180 stitches. In the third chain from hook, complete a double chain. Double chain in each stitch across. Chain 3 (represents first double chain) and double chain in each stitch. Repeat until the blanket is 80 inches long. Finish off.
When tackling a large project like this it is best to break it down to small squares or circles and then join the pieces together, much like a patchwork quilt.
If you have a King size bed, the Queen would be the same width across without dipping over the edges. If you have a twin bed, it will be roughly double the width. The length is the same.
Choose an afghan queen size blanket pattern, choose an afghan style(common names: Banded, Block, Cozy or Lacy). Then find your favorite yarn colors and styles, there are millions of colors and styles of yarn to choose from. Then you start the trial and error process.