Firstly you will need to ensure that the insurance of the motor vehicle covers the full period of the new road tax. If its not, you will have to call up your insurance company to request for an extension.
Next send your vehicle for inspection at one of the authorized inspection centres, if your vehicle is due for its statutory inspection. Weekend/Off-Peak/Revised Off-Peak Car must be sent for a number plate seal inspection before renewal of road tax. A certificate will be issued once your vehicle has passed its inspection.
Do ensure the vehicle has no outstanding warrants from HDB/URA/Traffic Police.