Once you have clicked 'Start' on 'Proceeds of sale investments', you'll be asked to upload a copy of a bank statement showing the amount being contributed. 'Proceeds of sale investments' is applicable to all purchase methods (mortgage/Help to Buy/cash) selected in the Pricing section. This step can be completed later by the buyer on their own platform, (simply click 'Buyer to complete later') or skipped entirely for the buyer to complete offline with their conveyancer (click on 'skip for this reservation'). The buyer will also be able to skip this step entirely from their own platform to handle offline with their conveyancer.
To upload, click on the upload box to select the file from your computer, or drag & drop a document from your computer to the upload box instead. Once you have uploaded the bank statement, click 'Save' and this step will be marked as done.