How is the load & trimsheet form sent to ACARS?
When the loadsheet arrives to the aircraft via ACARS it has to be acknowledged and printed. This is accomplished by the flight crew by selecting the proper screen and pressing the respective button or LSK on the MFD (Multi-Function Display for latest Boeing models) or in the MCDU (Multi-Function Control Display Unit for Airbus).

Along with the acknowledgement sent back to the system, goes the timestamp. Legally, this action is considered the wet signature. Then the sent ACARS form is printed in the aircraft printer, so the crew has a backup copy of the loadsheet containing in the header the "ACK" confirmation and the same timestamp sent to the system.

Every airline has different policies regarding the format of the loadsheet and even the procedure to accept and correct the values. that sometimes change in the last minute again, but this can give you a generic idea of the procedure.