craftgrrl | summer jacket type thing

okay!

basically, i saw a jacket type thing in a store ages ago, and i would like to try and make something similar. i sort of know what i want to do, i've never made my own pattern up before (well i made a tank top, that was relatively easy!) so what i am most worried about are the sleeves!

is this what a sleeve would look like to fit a back like the ones pictured?

that's folded out, the arrow.

i really don't know what i'm doing with the sleeves, but i'd like the final thing to look something like this -

i guess i'm just going to measure the back on something i already have, but i don't own anything quite like this! the material i'm using is a very lightweight cotton. any help would be MUCH appriciated xox