You cant!! They have to be older. Here is a link to help
http://www.austinsturtlepage.com/Art...softshells.htm Also Im going to suggest you look at this care guide,
http://www.austinsturtlepage.com/Car..._softshell.htm Once you start reading the requirements for enclosure you may not want to keep it. Direct quote from care guide "Recommended size for a single adult Apalone spinifera would be a 75 gallon aquarium as a minimum (90 or larger preferred) for a male. We don’t recommended large females be kept in conventional aquariums. They are too large & active and require extreme amounts of space. Stock tanks are an option, with a
minimum requirement of 400 gallons (600 gallon or larger preferred). This will provide adequate swimming space & assist the filtration in maintaining good water quality. For additional males, we recommend adding a minimum 75 gallons of tank space per additional turtle.
For females, add no less than 100 gallons of space per turtle.
With these space recommendations you may consider a swimming pool. Be sure you get one with great structural stability."
Just stuff to think about for the future