Welcome.
Here are 2 different methods of making QST's:
http://www.connectingthreads.com/tut...tars__D11.html
I prefer the 2nd method as you never have to deal with exposed bias edges.
I also always like to make these bigger and then trim them down to the exact size. I would start with 4.75" or larger squares.
(The actual calculation is finished size plus 1.25" - so anything over 4.25" would have some fudge factor in it. I personally like a lot of fudge factor. ;-) )
This way you're guaranteed of being able to cut them down to 3.5" unfinished after you make the QST.
Don't forget to post pictures when you're done.