These heat shrink connectors are far superior to the solder alternatives, and form a secure insulated join. If you will not be needing to disconnect the bullet joint at a point in the future then join male and female and then apply heat to shrink. However if you do need to disconnect then heat will be applied to both male and female prior to joining. Be careful not to apply heat around the female end or the heat shrink will form a seal around the metal that will not expand when the male is joined.

The male and female bullet heat shrink terminals fit together securely but you need to make sure the cables are crimped securely