Part one - content generation:
Using your choice of 3D editing program, align all your product pieces & their variations in the one scene. Export each variation for your products/pieces as separate glTF files.
If you have any pieces that have no variations, export them all as a single glTF file and import as a product with only a single variation.
Part Two - Uploading to Plattar
Create product 'group' to store all the pieces and variations of your product.
Create a new product for each of your product pieces.
Upload the variations into each of the product pieces. If you only have one variation, upload it to a product piece and it will not show in the configurator. If you want a blank variation, create a variation and don't upload any model to it.
For each of the variations, you can upload a thumbnail and a swatch. The thumbnail will be shown as the main image when the variation is selected, and the swatch will be the image of the button that the user clicks to select the variation.
You can set the order the pieces appear by dragging the handle on the left side of the variations, and you can set a default variation to be selected when the user first views the product.
Part Three - Bringing your product together
Create a new scene
Drag in the product component, and choose one of your product pieces
With the product selected, in the right hand panel under 'Group Controls', add the rest of the product pieces.
Click the 'Align Grouped Products' button after moving any piece to realign the rest of the pieces to match.
You can preview the result of your configurator by clicking the 'View Embed' button in the scene properties panel.