You'll first need to add a World Surface map layer.
Go to My Venues>Plans and if there is no World Surface created, click on 'Create New Plan' button and select World Surface from the options.
To add your site plan (or plans) as an overlay, you first need to have your site plan in geo-referenced PNG tiles format. You can do this by using third-party software such as Maptiler.
You can then upload these tiles by mousing over the right hand end of the World Surface row which will reveal an upload icon:
Click on this and you are presented with this screen, once you have clicked on the (+) icon under "Add tiles to world surface":
There are two options - either you can link the tiles from a self-hosted tile URL or you can switch to uploading (tile import).
Self-hosted tile URL: You'll need to input the URL, min & max zooms, latitudes and longitudes. You must give the import a name too - so remember to fill in the mandatory 'Tile import name' field just above the radio button. Then hit Save and the platform will then process the tiles.
Tile import: You'll need to have created a ZIP file containing folders of the tiles at each zoom level, plus the associated metadata.json file. Fill in the mandatory 'Tile import name' field before importing. And if the metadata.json file is present in the ZIP file you've uploaded, the information relating to the min & max zooms, latitudes and longitudes will be automatically assigned. Hit Save and the platform will then process the tiles.
You can repeat this process if you have multiple site plans (in multiple sets of tiles) as long as the location for each set of tiles does not overlap with any other set of tiles.
Once finished, you can check that the siteplan is visible in My Venues>Regions, ensuring you are on the World Surface (i.e. background map displayed). Then pan/zoom to the site location.