In some cases, you will find it difficult to add new fragments to your molecule file, for example if you already have structures cleaned in 3D. To add a new fragment to the canvas, follow these steps:
Choose the New substituent from the Insert menu.
Draw the structure in the new canvas. You can perform a 3D clean (Structure/Clean 3D) on the new structure to match to your original 3D molecule.
Click the Transfer button in the top left corner to return to the original canvas and place the new fragment.