In the UK....

If you are travelling with Garuda from the UK, your flight will be operated by Malaysia Airlines. Upon arrival at the airport, head for the Malaysia Airlines check-in area:
London Heathrow: Terminal 3, Area F
Manchester: Terminal 2

The recommended check in time for International departures is approximately 2 hours prior to the scheduled departure time, but check in opens 4 hours prior to departure time if you wish to check in early and miss the crowds.

These days security concerns are a high priority and delays can occur unexpectedly. So do check in early and do not risk having your plane take off without you. Most airlines close check-in 40 minutes before scheduled departure time.

Baggage....

Executive / Golden Club Class have a baggage allowance of 30kg per person.
Economy class has a baggage allowance of 20kg per person.

Infants (under 2 years and not occupying a seat) on Garuda operated flights, are entitled to 10kg baggage allowance. Infants flying on Malaysia Airlines operated flights have no baggage allowance.

Passengers on on Garuda operated flights are entitled to 7kg hand-luggage and it must be able to fit into the overhead lockers on the aircraft. All passengers flying on Malaysia Airlines operated flights have a hand-baggage allowance of 5kg.