Shortly before arrival at Terminal 1, fill out the Arrival Card form usually given out by your airline.
Tip: if you're arriving during peak hours, the Immigration queue can be really long. It helps if you get off the plane quickly. It also helps if you have a diplomatic or government passport, as there's a special lane for that passport type.
As soon as you get past Immigration, you may book your Uber or Grab or contact your personal pickup. There's free Internet access at Terminal 1, if you don't have an international data plan.
From the arrival area, head out into the pickup area:
Down there you'll stand in the area assigned to the first letter of your family name:
This is where you'll wait to get your Uber or Grab or personal ride. Inform your driver under which letter you wish to be picked. Pickup is expected to be quick here, as cars are not allowed to linger longer than necessary.