Slide the rubber boot over the wire, solder the connector, apply servisol silicon grease (maplin) to the rubber boot and the wire connector and slide on the rubber boot.
If the rubber boot is old it may well have gone hard, which may make it difficult. New Connectors generally fit together easily.
Also useful to pack silicon grease into the female connector, this will inhibit corrosion. Do not use petroleum jelly as its "petroleum" based and will damage the rubber
Diana