In the "tick" method , you need to set the y component of the deltaMovement to be positive (to rise) or negative (to fall). The exact value that you use will determine how fast.
//FallingBlockEntity uses the following values (when gravity is not disabled)
//by adding the velocity, it allows the entity to have smooth movement and allows directions other than straight down
this.setDeltaMovement(this.getDeltaMovement().add(0.0D, -0.04D, 0.0D));
Looking at the tick method of the FallingBlockEntity class it may give you a better idea of how it works.