Turn on Looping in sound forge
while the song is playing use 'I' key to mark the In point
use the 'O' to make the Out point.
LIsten to the loop and make sure that it loops perfectly. In if doesn't try th 'I' and 'O' key again or you can try to tweak you selection using the zoom tooland drag the edges of the selection.
Use the r key to define the selected section as a region
Press 'Z' to make sure that both ens are at a Zero Crossing
The region should look like this
Now if you have the selection perfect and the tempo of the song is constant you can try the acid looping tools
try these keys
shift + ,
shift _ .
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