I was given the start of a pencil plant. I had one but it didn't last long. Don't know what I did wrong!!! I've had it in a sand/ potting soil mix andin a sunny window with my other plants that are doing well. I water it about every 10 days. Thanks for you help!!

It might have been the sunny window and possibly dry soil for too long.
I can tell you what I do when I start this succulent.
I use 5"-6" cuttings, a small pot, not any bigger than 2 1/2" - 3" in size, clay or plastic is suitable. The potting soil is mixed with perlite so the soil is quick draining. Sand is ok, just not beach sand. The pot is then set out of the sun in indirect light while rooting and watered when the soil was very close to dry, which was usually once a week. After it rooted, which is quick, within a week or two, then it was put into some direct light for another week, then gradually into more light, after that, the cutting was watered when the soil was dry. Don't give up! You should try again.

Euphorbia tirucalli -pencil plant