When Flashing firmware one step is alluded, YOu need to restore the Firmware defaults settings.

This can be accomplished in a couple ways.

First Way.
Using a terminal that can send G-Code.
M502 reverts to the default "factory settings". You still need to store them in EEPROM
M500 stores parameters in EEPROM

Second Way.
Using the LCD with Marlin loaded
Store Memory and Load Memory


Explained

Load Memory loads the stored settings from EEPROM, and overwrites whatever is currently in the Controllers RAM
Store Memory stores the values to EEPROM, which makes them the default settings at boot.