2 ways to do it:

OPTION 1:

1. DoubleClick on Task to open up the Task Properties Page.

2. Click on the Predecessor Tab.

3. Checkoff the One or Many Predecessor you want to associate to Task.

OPTION 2:

1. From the Task List 

2. Click into the Predecessor Column

3. Type in the many predecessor you want to link separated by comma ie (3,4,5)

Hope this helps.