Follow this link published by the University of Chicago explaining how to complete the Trade-First subtraction algorithm.
http://everydaymath.uchicago.edu/educators/computation/sub_trade_first.shtml
Or - try this link, also by the University of Chicago.
Free Family Resources from Everyday Math
1. Click on the link below
2. Click on the Free Resources link in the bottom right corner
3. Click on Algorithms in Everyday Mathematics
4. Click 2
5. Click Subtraction
6. Click Trade-First Subtraction (focus)
This is a page you can use at home to practice the Trade-First Subtraction method.
Here is a website that allows you to move the base ten blocks between columns to visualize what it means to trade-first.
http://nlvm.usu.edu/en/nav/frames_asid_155_g_2_t_1.html?from=category_g_2_t_1.html
Trade-First subtraction App for your iPhone, iPad Touch, or iPad
http://itunes.apple.com/us/app/trade-first-subtraction/id351022585?mt=8
Ghost Bust Subtraction Practice
http://www.oswego.org/ocsd-web/games/Ghostblasters3/ghostsub3.html