For such a simple request, this is a real pain. Yes, using the patch is perfectly acceptable and easy enough - but sometimes 'set' commands are preferred for a number of reasons.
the easiest way I've come up with is to do this is to do the following
>show configuration | no-more | display set
copy your production config into excel
make your config changes, then from config mode
#show | no-more | display set
copy your candidate config into the same excel worksheet below existing config
from excel >data>remove duplicates
this will perform the delta and all that will be left will be the set commands for the candidate configuraiton. THIS WILL NOT INCLUDE DELETE COMMANDS
You can use any spreadsheet app to accomplish the same - if you want to learn something neat and perform this entire function on your juniper box then know that the junos supports awk at the shell level and it can be used to do this as well.
if you are not familiar with awk then i suggest googling around for its usefullness and examples.