For my pattern base, I use Jalie #2568 (low-cut hipster version). I'm sure there are tons of great patterns out there, but this one is my personal favorite. However, I have modified it in several ways. First, I added an extra inch to the top of the front and back pieces because the low-cut felt just a little too low for me. Then, I extended the lining an extra inch or so because it seemed too short, although this may be an anatomical anomaly in my case. I also widened the crotch area in general.
I first used the Jalie pattern with fancy new cotton interlock which is generally much stretchier than t-shirt fabric. So, when I use the pattern to cut from a tee, I add 1/2 inch to each side of the front and back pieces. This will definitely vary depending on the tee and on what size you are, but you will definitely want more give! The following pair I was cutting while talking on the phone and I accidentally forgot to add the extra fabric. After feeling upset for a minute, I realized that I could add strips to the side cut from a contrasting fabric and use a matching piece for the lining. I liked the result even more:
My final tip is to make sure to adjust the crotch area depending on the type of elastic you are using on the legs. If you are simply folding the fabric and sewing over it or if you are using picot trim, your pattern probably has this calculated into the measurements. However, if you are using fold-over elastic, you need to narrow the crotch or it may be too tight on your thighs. Again, this is all about individual tastes and sizes. If you're like me at all you'll probably make lots of mistakes but just keep trying because it's totally worth it in the end. Speaking of mistakes, always soak your elastic in warm water unless it's very light-colored! Anyway, I hope these tips are helpful.
Thank you everyone for your awesome, inspiring posts!