I happen to have a pile of old basic long sleeved shirts I got at Target when I was nursing Baby. Read: they are ridonkulously shapeless on me now, so they just sit in my dresser.
Take basic long sleeved shirt:
Cut off the bottom hem.
Cut up the middle of front of shirt.
Just eye ball it! It is jersey it will roll! No problem!
Cut sleeves to desired length so they will roll too. (And so it looks intentional and not like you just cut up a shirt.)
My shirt had a crew neck collar. I just trimmed off the ribbed fabric but left the stitching around the neck opening.
You could cut it off. It doesn't matter!
Put on the cheap outfit you bought at Target because it was 90 out and you don't have any tank tops or shorts that fit. Elastic waists are in! yay!
Then throw on your new cardigan so you don't feel so trashy at the grocery store!
If you are the kind of person who can pull off the tied up look you can do that too:
Of course I am not that kind of person, so I do beg your forgiveness for exposing you to that.
Alright, have at it.
White shirt... I'm coming for you.