Downtime and Levelling Up

All characters begin at level 0 with no assigned Class and no special abilities or bonuses. After a period of gameplay though, they may move up to level 1 and later increase to the 2, 3, 4 and so forth. In-game, this equates to a period of at least two weeks in which the character is learning and practising new skills and abilities. After this period, make sure to update your character with any new class bonuses and abilities, add any gained proficiencies and roll one hit dice (appropriate to the character's species) and increase their maximum hit point total by the result.

Multi-classing

After a character has taken one or more levels in a class they may, if their player wants them to, take their next level in a different class. In this case they retain all their levels, abilities and bonuses in the first class and then gain one level in their new class, getting any abilities and bonuses appropriate to that level of that class too.

For example: a level 3 Pilot could, when levelling up, take 1 level as an Engineer. In this case, they still increase their maximum hit points per a roll of their Hit Dice, then they note down any abilities and bonuses for a level 1 Engineer.