After completing your base row of doubles, turn the work and make three chains to count as first stitch.
Wind yarn around hook(YO) and insert the hook between the first and second dc from front to back.
Take hook behind the second dc then bring it to the front between second and third dc.
Wind yarn around hook(YO) and draw it through. You now have three loops on hook. Complete the double crochet in the usual way. This is called FPdc.
Work around next two dc in the same way.
These first four stitches (counting the turning chain at edge) will form part of the first vertical block.