To Install with the defaults. 
This is the best method if you don't want to manually edit files.

Step 1 - Change to /home/pi and Download the install script

pi@raspberry:~ cd ~

pi@raspberry:~ wget https://raw.githubusercontent.com/Theoi-Meteoroi/GPIO-pca9548/master/mux-install.sh

pi@raspberry:~ /bin/bash mux-install.sh

After reboot, you should see something like this in dmesg output:

[    2.168710] systemd-udevd[137]: starting version 215

[    2.527337] i2c-gpio i2c@0: using pins 23 (SDA) and 24 (SCL)

[    2.796954] i2c i2c-3: Added multiplexed i2c bus 4

[    2.799319] i2c i2c-3: Added multiplexed i2c bus 5

[    2.801690] i2c i2c-3: Added multiplexed i2c bus 6

[    2.803925] i2c i2c-3: Added multiplexed i2c bus 7

[    2.808755] i2c i2c-3: Added multiplexed i2c bus 8

[    2.809316] i2c i2c-3: Added multiplexed i2c bus 9

[    2.811035] i2c i2c-3: Added multiplexed i2c bus 10

[    2.812586] i2c i2c-3: Added multiplexed i2c bus 11

[    2.812604] pca954x 3-0070: registered 8 multiplexed busses for I2C switch pca9548


This is with the default i2c address of 0x70.  You can pass another address in the declaration:

dtoverlay=i2c_gpio-pca9548,addr=0x71

You can also list out the configured I2C bus instances with

pi@raspberry:~ sudo i2cdetect -l