See the datasheet sect 12.8 (this can be different for each device)

To map the input use the register shown in 12-1 'xxxPPS peripheral input selection' along with Table 12-1
To map the EUSART RX, xxxPPS=RXPPS... that's the register you need to set.
Look back at 12-1 to get the PORT and PIN setting.
For example, to select PORTB pin 4 the setting would be '--001100', so
RXPPS = $0C

To map the output use the register shown in 12-2 'RxyPPS output source selection register'
along with Table 12-2. 'x' is the PORTx (ie 'A','B', 'C') and 'y' is the pin '0'-'7' (ie 'RB5')
To map the EUSART TX, the output signal in Table 12-2 is TX/CK which shows RxyPPS = 100100, so
RB5PPS = $24