The Usual measurements are 1" (front) and 2" (reverse) .... but your sashing/joining strips can be any width you want
Using the 1" width ....
you would trim your quilted block ... top, wadding and back ... to 12 1/2"
Lay your 1" width strip RST along the edge of the top of the block,
the 2" strip (folded in half WST along its length) along the edge of the reverse of the block ...
Sew 1/4" seam from that edge.....
Press the 1" strip back on itself (leave the Reverse strip as is, and lie RST with the next block trimmed as above
Sew a 1/4" seam along the edge
This will leave you with 1/2" strip of fabric joining the two blocks and two 1/4" edges that sit nicely together underneath the sashing/joining strip.
But your strips could be wider ..... you just have to remember to trim your blocks and sew your seams accordingly ....
....
It is so much easier than it sounds ... If only you could come and see the process
I feel a Tut coming on ... Bear with me.