First quarantine the newbie to make sure there are no illnesses and double check that it is indeed another female.
Then the best way to introduce them is to have them meet in a large neutral area that doesn't smell like either pig. Sit back and patiently let them get to know each other. This will take time and should not be rushed.
You can include a couple water bottles, hay piles and veggies spaced out in the area. Avoid hideys or areas where a pig can get trapped. I also always make sure I have a towel handy in case I have a nasty fight and need to use it to separate them quickly (don't use your bare hands!).
If they get along, make sure to put them in a clean cage that is big enough for 2...the bigger the better.