Very simple tank top patterns. Use the back of a t-shirt that fits you well to get the armhole shaping - then add at least two inches to the sides to make in regular cotton. Choose something very light-weight that will gather softly and breathe well in hot weather. To make the garment easy to put on and take off (as there are no buttons or zippers) make sure the bottom hem distance is wider than the distance across your shoulders.
For this style make the front and back sides alike, then estimate how large the side panels should be by holding up the front across your chest. Sew side panels to front and back on both sides.
Lay flat so the armholes are in the center. Trace armholes for facings on plain fabric. Cut two facings. Zig-zag or serge bottom edges of facings and pin right-sides together with the armholes of the garment. Sew edge and turn right-side out. Sew 1/2 inch from edge to secure armhole facing down. Top edges of facings will be turned under in the elastic casings, so don't worry about finishing them.
Fold top edges of front and back sides down to form casings for 1/4 inch elastic. Sew casings down and use a safety-pin attached to the end of the elastic to draw the elastic through the casing. Sew the elastic down firm at one edge of the casing and pull other end (on safety pin) to gather the front to nearly 3/4 the original distance. Sew the elastic down firm at the other end of the casing in the 'elasticized' position. Repeat for back of garment. Sew bottom hem all the way around. Make straps for shoulders as long skinny tubes turned inside-out. Measure over shoulder to find the right distance for the straps, add 1 inch to each side to give enough room to turn under raw edges and tack in place at 'points' of either edge above armholes.
NOTE: I made this out of two old items, a pillowcase and a curtain sash that were both in a box of remnants I received. I used whatever thread was in my bobbins as I did not care about matching thread colors on this first run.
I really wanted more tank tops - but what you get in the stores are very overpriced for what they are, knit t-shirts that will tear up in the washing machine and start to tear out at the seams with frequent washings. Thus - this. It will work perfectly for what I've intended - walking in our yard, down to the lake and staying cool in the house in the summer and/or as a nightgown. It feels nice to be able to whip up something basic and decent quality out of what I have on hand and not have to pay money for something that will not last very long. Plus, because I made it - I can reproduce it, whenever I get some time.