You will need:
Put the top inside the legging then pull the legging up till desired height.
Once your happy, mark the height with a pin.
Tip: Its better to wear the top inside out.
Cut the extra fabric off. Don't forget about your seam allowance.
Optional: Draw stichting line with a chalk on the inside of the top.
I removed the elastic to get more length, so I could get the height that I wanted. This step is time consuming and it also depends how high waisted you want your jumpsuit to be.
You can skip this step if you want too.
Turn your top inside out and place it over the legging. Then pin them together and sew all the way around with a straight or a zig-zag stitch.
Make sure to stretch the fabric when you sew.
Cut off the extra seam allowance if you have and lastly serge the edge