From Planet Computers
Once you downloaded the Base firmware, unzip it to reveal the firmware folder, for example Gemini_x27_08102018. Next, simply copy your custom scatter file into that folder. Finally, use the Flashing Tool to write the updated Android firmware on your Gemini. You can refer to the [http://support.planetcom.co.uk/index.php/Android_Flashing_Guide Android Flashing Guide] to download and install the Flashing Tool for Windows and for Linux.
<span style="color: #252525; font-family: 'Source Sans Pro', sans-serif; font-size: 15.008px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: #ffffff; text-decoration-style: initial; text-decoration-color: initial; display: inline !important; float: none;">Once installed, you will need to configure it using the custom scatter file inside the Android base firmware. See the following screenshot for reference
The most important thing here is to untick the userdata partition, which contains your data, so that your information is not deleted from the device. If you accidentally flash the userdata partition as well, then all your data will be lost. If you have a custom multi-boot Gemini as used in the screenshot, then most likely other partitions will already be unselected, this is fine.