Fo step 1, use the attached drawing. Keep in mind that 4051 is partially analog circuit so the I0-I15 and Out1/2 sugnals can be from -4,5V to +4.5V. Address lines A, B, C and di are of 0-5V TTL/CMOS levels.
(Summing Opamp at the output is not required, you can connect both pins Out1 and Out2 together..it is just in case you need to feed this signal somewhere else).
For step 2, just feed signal without opamp to both pins Out1 and Out2, and depending on the A, B, C and D address lines you will pick up this signal at one of I0 to I15 lines, this time working as outputs.
Good luck and regards,
IanP