From an appropriate location:
mkdir replica-set
mkdir -p replica-set/rs0-0 replica-set/rs0-1
mongod --port 27018 --dbpath replica-set/rs0-0 --replSet rs0 --smallfiles --oplogSize 128 &
mongod --port 27019 --dbpath replica-set/rs0-1 --replSet rs0 --smallfiles --oplogSize 128 &
mongo localhost:27018