I2C slave backends behave like standard I2C clients. So, you can instantiate them as described in the document instantiating-devices.rst. The only difference is that i2c slave backends have their own address space. So, you have to add 0x1000 to the address you would originally request. An example for instantiating the slave-eeprom driver from userspace at the 7 bit address 0x64 on bus 1::
# echo slave-24c02 0x1064 > /sys/bus/i2c/devices/i2c-1/new_device
Each backend should come with separate documentation to describe its specific behaviour and setup.