How to create the Item xml file

Just create an empty text file (which has the extension ".txt" by default) and change its extension by renaming it {ItemName}.Item.xml (for example: Slope_Base.Item.xml).

Open this file in your favorite text editor (for example: Notepad++), and copy-paste the following template:

<Item Type="StaticObject" Collection="Stadium" AuthorName="YourAuthorName">
  <MeshParamsLink File="Mesh/ItemName.MeshParams.xml"/>
  <Waypoint Type="Checkpoint"/>
  <Pivots>
    <Pivot Pos="0 0 0"/>
    <Pivot Pos="0 0 -1.5"/>
    <Pivot Pos="0 0 1.5"/>
  </Pivots>
  <GridSnap HStep="8" VStep="2"/>
  <Levitation VStep="2" GhostMode="true"/>
  <PivotSnap Distance="0"/>
  <Options OneAxisRotation="true" ManualPivotSwitch="true"/>
</Item>

Then adapt it to your situation, according to the following explanations:

Item tag is mandatory
attributes:
- Type is mandatory and should not be changed ("StaticObject")
- Collection is mandatory and should not be changed ("Stadium")
- AuthorName should be changed to the name you want to be called in the Trackmania creative community :)
sub-tags:
- MeshParamsLink tag is mandatory
- attribute:
  - File: Facultative. The relative path of the .MeshParams.xml next to the .fbx file. If omitted, the .MeshParams.xml will be expected to have the same name as current .Item.xml file and to be located in a sub-folder "Mesh".
- Waypoint tag is facultative. Use it only if you are making an item which is a special waypoint of the map, like a checkpoint or a finish line.
- attribute:
  - Type: mandatory. Must be one of the following: "Start", "Checkpoint", "Finish", "StartFinish" ("StartFinish" is used for multilaps).
  - Note: this is not enough to make a valid waypoint item. See section How to make a waypoint item for more information.
- Pivots tag is facultative. See section Pivots and placement parameters for more information.
- sub-tags:
  - Pivot: put one Pivot tag for each pivot you want to add to your item.
  - attribute:
    - Pos: Position of the pivot, in the base of your mesh. (0,0,0) would be the same origin as in the fbx file.
- GridSnap tag is facultative
- attributes:
  - HStep: Facultative. A positive real value in meters (0 by default). See Grid Horizontal Size in section Pivots and placement parameters for more information
  - VStep: Facultative. A positive real value in meters (0 by default). See Grid Vertical Size in section Pivots and placement parameters for more information
- Levitation tag is facultative
- attributes:
  - VStep: Facultative. A positive real value in meters (0 by default). See Fly Step in section Pivots and placement parameters for more information
  - GhostMode: Facultative. A boolean value: "true" or "false" ("false" by default). See Ghost Mode in section Pivots and placement parameters for more information
- PivotSnap tag is facultative
- attribute:
  - Distance: Mandatory. A positive real value in meters, or the special value -1 (-1 by default). See Pivot Snap Distance in section Pivots and placement parameters for more information
- Options tag is facultative
- attributes:
  - NotOnItem: Facultative. A boolean value: "true" or "false" ("false" by default). See Not On Object in section Pivots and placement parameters for more information.
  - OneAxisRotation: Facultative. A boolean value: "true" or "false" ("false" by default). See Yaw Only in section Pivots and placement parameters for more information.
  - ManualPivotSwitch: Facultative. A boolean value: "true" or "false" ("false" by default). See Switch pivot manually in section Pivots and placement parameters for more information.
  - AutoRotation: Facultative. A boolean value: "true" or "false" ("false" by default). See Auto Rotation in section Pivots and placement parameters for more information.

You should get something like this:

<Item Type="StaticObject" Collection="Stadium" AuthorName="NadeoSamples">
  <MeshParamsLink/>
  <GridSnap HStep="8"/>
  <Options OneAxisRotation="true"/>
</Item>