Example map: https://trackmania.exchange/maps/45905/platformbase2slopetransitions
Instruction: unzip the downloaded file into your items folder (Trackmania\Items) and keep the folder structure or create an additional subdirectory named "iNDEX" and extract the ZIP-file here.
Intended folder structure for this item set: "Documents\Trackmania\Items\iNDEX\PlatformBase2SlopeTransitions"
Download the following item into the subdirectory "iNDEX":