Only used EC5- this is my method;

Strip and tin the wire, make sure it still fits in the connector... Heat and tin the inside of the connector, push the wire in while maintaining the heat and add solder as required...

The most important thing is to make sure the outside of the brass contact is clean, if there's a blob of solder on it, it'll never go in the whole... Then get a small screwdriver push the connector on it and lean on it- takes a fair bit of force to get it to click into position but once it's in, it's not coming out.