Easiest way is to go to 'Cuts/set up custom cut' in SB3 and paste your code in the middle clear section, put a name at the top as stated in the instructions and then Save as with a number of a open slot. Be careful what one you choose so that you don't over write one that you or SB uses.
Close SB3 and restart it, and it should show in the list unless it's a high number, then it may not be on the list, but you can still call it up with CN and the number that you saved it as. Example, Custom13.sbc would be CN13.
You can also just open one of the customXX.sbc files, cut and paste your code, change the description and save as to another number.
“Participating in a gun buy back program because you think that criminals have too many guns is like having yourself castrated because you think your neighbors have too many kids.” Clint Eastwood
Those who hammer their guns into plows, will plow for those who do not - Thomas Jefferson