It's fairly simple if the custom planet is using the default SE ore placement, in the Data folder for the mod you should find a file named [planet name].sbc

Find the section with <OreMappings>[resources defined here]</OreMappings> in that file, then replace with the code below

<!-- Default resources -->
<Ore Value="1" Type="Iron_02" Start="12" Depth="8" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="8" Type="Galena_01" Start="22" Depth="4" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="12" Type="Copper_01" Start="14" Depth="8" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="24" Type="Nickel_01" Start="14" Depth="6" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="48" Type="Silicon_01" Start="10" Depth="8" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="60" Type="Petroleum_01" Start="14" Depth="8" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="72" Type="Cobalt_01" Start="18" Depth="6" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="84" Type="Lithium_01" Start="20" Depth="6" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="108" Type="Gold_01" Start="26" Depth="4" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="120" Type="Magnesium_01" Start="22" Depth="6" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="144" Type="Wolframite_01" Start="24" Depth="8" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="152" Type="Titanium_01" Start="28" Depth="4" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="156" Type="Aluminium_01" Start="16" Depth="6" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="168" Type="Nitre_01" Start="20" Depth="6" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="132" Type="Ice_01" Start="13" Depth="7" TargetColor="#616c83" ColorInfluence="15"/>
<Ore Value="180" Type="Silver_01" Start="21" Depth="3" TargetColor="#616c83" ColorInfluence="15"/>
<!-- Default resources -->

If it has it's own custom ore placement you will need to edit the 6 material map files (Data\PlanetDataFiles\[planet name]\*_mat.png), first the blue channel for the existing ores will need to be reset to 255, then overlay my add.png (included in 'Scarce Resource'), follow the steps above and replace with

<!-- Scarce resources -->
<Ore Value="240" Type="Sulfur_01" Start="32" Depth="3" TargetColor="#616c83" ColorInfluence="15"/> <!-- Blue channel = 15 (0) -->
<Ore Value="225" Type="Silver_01" Start="32" Depth="8" TargetColor="#616c83" ColorInfluence="15"/> <!-- Blue channel = 30 (24) -->
<Ore Value="210" Type="Nitre_01" Start="28" Depth="6" TargetColor="#616c83" ColorInfluence="15"/> <!-- Blue channel = 45 (72) -->
<Ore Value="195" Type="Iron_02" Start="26" Depth="12" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 60 (120) -->
<Ore Value="180" Type="Nickel_01" Start="28" Depth="8" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 75 (120) -->
<Ore Value="165" Type="Silicon_01" Start="26" Depth="10" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 90 (120) -->
<Ore Value="150" Type="Cobalt_01" Start="30" Depth="8" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 105 (96) -->
<Ore Value="135" Type="Magnesium_01" Start="42" Depth="6" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 120 (96) -->
<Ore Value="120" Type="Wolframite_01" Start="30" Depth="8" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 135 (72) -->
<Ore Value="105" Type="Petroleum_01" Start="30" Depth="10" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 150 (96) -->
<Ore Value="90" Type="Copper_01" Start="26" Depth="10" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 165 (72) -->
<Ore Value="75" Type="Aluminium_01" Start="28" Depth="8" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 180 (96) -->
<Ore Value="60" Type="Lithium_01" Start="34" Depth="6" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 195 (72) -->
<Ore Value="45" Type="Titanium_01" Start="42" Depth="4" TargetColor="#616c83" ColorInfluence="15" /> <!-- Blue channel = 210 (48) -->
<Ore Value="30" Type="Gold_01" Start="46" Depth="4" TargetColor="#616c83" ColorInfluence="15"/> <!-- Blue channel = 225 (48) -->
<Ore Value="15" Type="Galena_01" Start="38" Depth="6" TargetColor="#616c83" ColorInfluence="15"/> <!-- Blue channel = 240 (48) -->
<!-- Scarce resources -->

Have a look a https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=566387835 which explains the necessary steps very clearly.