Power output from roomba and ground go to buck converter. Then output and ground from buck is connected to vin/v+ of the esp if it has 5V power input or to the 3v pin. If the buck converter only outputs a certain voltage connect it to the appropriate pin. 5V is preferred if possible for lower amperage.
The roomba output from 11-something to 20-something (not really sure the exact values) volts. Anything above 3/5 v (again depending on the power input you have on the board) will kill the esp immediately.
If you look at my picture, I have a 3v-5v convertor with 3 wires to the roomba for data. And a power convertor 6-36V to 5v switching power supply. This means the esp sees 3v on its data pins and 5V on power pin while the roomba gives out any voltage it wants for power (fluctuates with movement/charging) and it sees 5V on its data pins.
If you give more than 3v on data pins and more than 1V on analog pin or more than 5V on the 5V input pin or 3V on the 3V power pin you kill the board.
Also all grounds should be connected between them.
I am a self-taught electrician so take all this with a grain of salt. While I haven’t fried anything in the past 10 years I lack a good training in this stuff