As it is shown in the photo with 4 stars across and 3 down ... I'll explain with those dimensions.
Note: This is just for sashing INSIDE the blocks ... does not include a border.
Also note: This is assuming that block size of 12" you gave is the FINISHED size, not the size the blocks are currently (before sashing).
Your going to need 6 2.5" X 2.5" blocks of yellow - a fat quarter is more than enough
You will need 17 2.5" X 12.5" strips of sage green for the sashing. 17 X 2.5 = 42.5 which is good because you can cut 2.5" strips across the WOF (so every strip will be with the length of the fabric). If your store only sells in 1/4 yard increments you will need 1/2 yard. If your store sells in 1/8 yard increments you can save some $$ and only buy 3/8 yard (13.5").
Now, math is not my best subject so you might want to wait until someone else chimes in and hopefully we have the same answer
