To my knowledge two basic sestina formulas exist, normal and double sestina. Normal sestina has six stanzas, each consisting of six lines, and a three line ending stanza called envoy. It is easy to find a formula for the normal sestina, but there appears to be several formulas for a double sestina. My first try with a double sestina was to use John Ashbery's formula from a sestina in Flow Chart. Ashbery used words from Charles Swinbyrne's The Complaint of Lisa. First I used the same word pattern, replacing Swinbyrne's/Ashbery's line endings with my own, but I was unable to figure out the logic behind the formula. The normal sestina format can be deducted in a beautiful way using a spiral (see figure 1). Arabic numerals in the figure indicate ending words of a line, roman numerals indicate the stanza. When you use the spiral to the line endings of the first stanza, you get the repeat pattern for the line ending words for the second stanza. When you apply the spiral again for the second stanza, you get the repeat pattern for the line ending words for the third stanza. Repeat this until you have six stanzas.
Please note that a proper sestina requires also the envoy, of which there are different patterns. Wikipedia article on sestina.
My previously posted collage/cut-up double sestina uses Swinbyrne's/Ashbery's formula.