Character advancment in a FlexD6 system is very simple. When it seems appropriate, select a new skill or add 1 to an existing skill. Skills max out at 3 points.
In the base rules, there is no XP or timing limitations on advancement. It is up to the Facilitator and players to work out when advancement happens. For some guidance, consider giving the characters a bump every couple of sessions.
Also, to keep the system balanced, the characters Health does not increase as they advance. Their skills advance, making it more likely they will succeed and less likely they will take damage, although, this system is still very deadly for advanced characters. To keep that danger level up, characters health remains static.
Another way to advance the characters is to give them more advanced items. Either they can find the items or spend their coins to improve an item they have. A +1 Skill die weapon will come in handy. See the Items section for more information about advanced items.