Basically you connect the Tx pin from one end to the Rx pin of the other end and vice versa, then Serial.print() from one end and Serial.read() on the other when Serial.available() indicates that something has been received
If you are going to use a Uno as the receiver then you would be wise not to use its 0 and 1 pins because they are used by its hardware Serial interface. Instead consider using SoftwareSerial on the Uno on 2 other pins of your choosing