First, measure your bust and around both of your legs, mid thigh. You can then cut your elastic to these lengths, as it will stretch.

Then, take your bust measurement and add 1ft. Cut material horizontally.

<< Thats material, not cheese!

Decide on your dress length, and add 2 inches for hems for top and bottom.

Now, this bit is kinda tricky!
You need to pin the end of your elastic one inch down from the edge of your fabric, at each end. (Bust and thigh bits)
Then, securing one end under the sewing machine, pull the elastic stretched and sew along. Be careful you don't pull it out of the machine!
Dont take an axe to your machine if it all goes wrong! just unpick and start over.

Hopefully, you will have got your timing right and come to the end of your elastic at the same time as your fabric. This is important! Do both sides, and then it's just a case of turning your dress inside out and sewing it together! Make sure the elastics are sewn together to create strong support for the dress. Et voila! Sorry, not very clear instructions! :)