A basic working example of some XML.
You can find a written guide here: https://wiki.guildwars2.com/wiki/User:Lady_Elyssa/Guide_to_GW2_TacO_Markers_and_Trails
<OverlayData>
<MarkerCategory Name="mymarkers" DisplayName="My Markers" fadeNear="2500" fadeFar="5000">
<MarkerCategory Name="example" DisplayName="Example Trail" texture="Data/Images/trail.png"/>
<MarkerCategory name="circle" DisplayName="Toggle Circle" iconFile="Data/Images/circle.png"/>
<MarkerCategory name="heart" DisplayName="Toggle Heart" iconFile="Data/Images/heart.png"/>
<MarkerCategory name="square" DisplayName="Toggle Square" iconFile="Data/Images/square.png"/>
<MarkerCategory name="triangle" DisplayName="Toggle Triangle" iconFile="Data/Images/triangle.png"/>
</MarkerCategory>
<POIs>
<Trail type="mymarkers.example" trailData="Data/example.trl"/>
<POI MapID="15" xpos="-200.086" ypos="28.6689" zpos="331.556" type="mymarkers.triangle" GUID="AVE8BmBQdkG32qvKSxkm6w=="/>
<POI MapID="15" xpos="-194.003" ypos="28.6396" zpos="338.98" type="mymarkers.circle" GUID="RT3v6Tr5gE6yyav0dFO4uQ=="/>
<POI MapID="15" xpos="-186.742" ypos="28.6003" zpos="331.901" type="mymarkers.heart" GUID="As9RB0soykqn4NsqA1abvA=="/>
<POI MapID="15" xpos="-193.448" ypos="28.6712" zpos="324.628" type="mymarkers.square" GUID="BV3HKuCr6UapNoZMT2R1JQ=="/>
</POIs>
</OverlayData>