Here's what MLD's site says:
"The very first time you set up your Innernet and Mid will take about 10-20mins to get the pitch correct. After the corner bungee loops are tied and adjusted the InnerNet and Mid will mate and set up quickly.
1: First set up you Mid.
2: Then insert the Innernet and clip it to the Mid's inside apex loop and adjust center pole.
3: Note the distance from each corner of the Innernet to the outer Mid and the length of bungee loop needed to reach the corners.
4: Tie 3/32" bungee loops ( cord supplied) to the outside Mid's corner tieouts loops behind the linelock and then clip the bungee loops to the Innernet corners.
5: Adjust the tension on the Innernet floor by lengthening or shortening the bungee loops.
DO NOT OVER STRETCH THE INNERNET CORNERS
NOTE:
DO NOT OVER STRESS THE CORNER TIEOUTS.
ONLY PULL AND STAKE THEM TIGHT ENOUGH TO GENTLY FORM THE FLOOR SHAPE.
THE CORNERS ARE TRIPLE STITCHED AND ARE STRONG BUT WILL NOT WITHSTAND EXCESSIVE FORCE.
CORNER RIP OUTS ARE NOT COVERED BY WARRANTY.
NOTE: We highly advise you also use McNett ZipCare on the zipper when new and then every 30 days of use for a long life.
The InnerNets are sized to fit under all the MLD mids that are the same size or larger and can fit under many other shelters and tarps or they can be set up on their own.
For example, the Solo Innernet will fit the SoloMid, DuoMid, Speed Mid and SuperMid and can be set up under most Duo Tarps and the Solo Trailstar and full size TrailStar. (The Super InnerNet will only fit the SuperMid- It is too tall for the SpeedMid.)
The short elastic corners tieouts loops with hooks on the InnerNets are not long enough to fit any of the the shelters exactly in order to give you various pitch options in the Mid- higher off the ground or low.
The hooks can be moved up or down to create the best angle for various pitches- just slide the small hook higher or lower to change the angle.
In most cases when an Innernet is used under a Mid, you will need to add loops of bungee to extend the corner stake points for your particular shelters. A length of 3/32" bungee is supplied and will be cut into four loops to fit your setup.
The bungee loops can be staked down with the Mid stakes OR can be to tied to the Mid's corner tieouts OR can be staked independently.
NOTE:
DO NOT OVER STRESS THE CORNER TIEOUTS.
ONLY PULL AND STAKE THEM TIGHT ENOUGH TO GENTLY FORM THE FLOOR SHAPE.
THE CORNERS ARE TRIPLE STITCHED AND ARE STRONG BUT WILL NOT WITHSTAND EXCESSIVE FORCE.
CORNER RIP OUTS ARE NOT COVERED BY WARRANTY."
I think you got your inner second hand, right? If so, they may not have included the bungie cord; if so, you may need to get some from a craft or sewing shop. Once you have it set up in your Duomid, you can just leave it there.