Prepare your bar/patterns in pattern mode
switch to song mode
press record
change patterns anytime you want, turn volumes up, tweak filters etc.. (you don't have a limit in time here!). Then enable the LOOP thing,a nd set the range from pattern 1 to the end of your song (exemple 300).
If you do an export file>export loop, you will export the whole 300 patterns

check the tutorial. If you have any question do not hesitate.
Have fun!