if you decided to use the corner blocks ( I think they are WAY cute) then how I would recomend stitching them together is blocks at a time and not strips. strips are harder to get lined up and more of a pain to unpick! even with the sashing it can be difficult to line up!
take your Ispy square and sew a black strip to it, then take your corner square and sew it to the end of another black strip. then sew the small square/strip to the top of the large square/strip.
do this with each Ispy block, lay them out to make sure your pattern matches up ok, then sew two together, then two more, then those two to make four, and so on... you will need to sew more on at some point for one side and the bottom, but that will be easier after you start getting the hang of how they need to go together.
at least thats how I do it, I hope it makes sense.