To create your own basic tank pattern is rather simple. The first thing you need to do is select an existing tank from your wardrobe that you know fits your body well. Another thing to consider is stretch - if you are using a non-stretch fabric for your tank, make sure the tank you choose to pattern is also non-stretch and vice versa.
Most of the things I make use a non-stretch fabric like cotton broadcloth, chiffon, silky polys (yes I know polys are evil but I'm not confident enough in my sewing skills to buy silks nor do they even sell silks around here) so here I've chosen an old Ann Taylor shell in XS to pattern off of. Fold the tank in half the long way and align the fold with the edge of some tissue paper (or any other large sheet of paper you have around the house).
So that's essentially the basic tank portion after the sides and shoulder have been sewn together and the tank has been hemmed. For finishing the collar and armholes - you can either do it like your hem (fold inwards and stitch) or you can make a facing. A facing is like a mini lining that you sew for collar and armholes so that there is no visible hemming. To make a facing, re-trace your front and back patterns but this time you will only need the top portions as indicated by the red dotted lines below.
Now for the bow ruffle embellishment, you will need to cut the following:
2 pieces of 7x7.5"
4 pieces of 6x6.5"
2 pieces of 6x8"
1 piece of 1x5"
For items 1-2, pair up the matching pieces into sets of 2 with right side of the fabric in the middle. Sew three out of the four sides together and flip inside out. This will give you a rectangle with an open top. Do this for all 3 pairs of ruffles.
For item 3, you can essentially do the same as above but you'll want to at least partially sew 3/4 of that open top before you flip it inside out. Then you'll want to blind stitch the rest of the top. Here's a nifty video on how to blindstitch.
For item 4, fold in half and sew the long side. Flip inside out (using a chopstick or something similar).
Time to attach the ruffles! Use item 4 to wrap around item 3 to make a bow. Secure the bow by stitching item 4 into a loop around item 3.