If you are new to Foldit, here are detailed instructions to get you started:
http://fold.it/portal/node/988864

HINT TO EASILY MAKE A SHEET:

The rebuild tool takes structure assignment into account, so you can set the secondary structure to Sheet and then Rebuild. Foldit will then make a sheet out of that region.

ANOTHER HINT: If you are having trouble with this puzzle, try using the "Align Guide" tool:

By right clicking on a region of the protein (not the guide) it will pull up a menu with 6 tools. The bottom right one is "Align Guide". Clicking on it will align the native guide to the region of the protein you selected.
This will help you see what sections on your protein correspond to the guide!

Working with the guide is much more user friendly when you use the advanced viewing options.
In the game click on Menu->General Options and select "Show Advanced GUI" and click Done.
Click on View, turn on 'Show Outline,' set the background color to white, and view stubs on the sidechains, view the guide, and set color relative to guide. Turn all the other view options off."

For even more info: http://foldit.wikia.com/wiki/Tlaloc%27s_Strategy#Technique_for_Matching_a_Guide