All Nipon Airways Boeing 777, from Eddie's collection
I've been asked more than once for my thoughts on international slots, so here they are. I've collected information on slot procedures in many places around the world only to decide the effort was fruitless. The procedures are constantly changing and you would be well advised to check an airport's procedures even if you've been there recently. If you have a better approach please let me know, "Contact Eddie" below.
The EU seems to have its act together and I've laid them out separately: International Operations / Slots (European Union).
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
We have a world class dispatcher and we make liberal use of international handlers (because it always pays to have someone in country on your payroll) but we as pilots have to know about slots if we travel to some of the more busy airports in the world. A "slot" is simply a position in line to use an airport, its facilities, or to simply park there. The busier the airport, the more likely they are to use slots. Over the years I've found Nice (LFMN), Frankfurt (EDDF), Geneva (LSGG), and Changi (WSSS) to be particularly challenging. But they aren't alone. You need to think about slots early in the trip planning process.
There are a variety of request forms and procedure, you need to call the airport to be sure. The standard procedure used to be picking up a phone and getting a reservation number to recite when challenged. These days there is often a website procedure. Here are a few examples:
Quite often a slot is requested and adjusted exclusively through a ground handling agent or can be adjusted directly with an airport authority available on an airport frequency, such as ground control or a specified slot coordinator. No matter the format, the actual submission can be made by SITA, AFTN, fax, phone, e-mail, or a dedicated website. You should call someone at the airport well in advance so you know who to deal with when the time comes. Generally speaking, they will prefer you deal with a ground handler for set up and revisions.
There are variations in minimum and maximum lead times. Some airports require as much as 3 months notice where others can deal with you in only a few days. Some will not talk to you any earlier than seven days in advance.
Once a slot is issued, it is valid plus or minus a specified allowance. At Nice, for example, the slot is good -5 / +10 minutes.
Once a slot is issued, it is generally a good idea to make it. If you can't make it, you should notify the slot authority as soon as possible so the slot can be reassigned to someone else. For a landing slot, work with air traffic control. For a departure slot, work with your handler or the airport authority. The penalty for missing a slot can vary from a simple delay to fines and even a ban from the airport for a specified period.
At some airports revisions are simply not allowed, such as at Beijing (ZBAA). At others it is simply a matter of asking ground control for an adjustment, such as at Luton (EGGW).
If your slot request is denied you usually have options. You can ask for a less favorable time period. If you can't get into the airport during business hours, perhaps late at night or early in the morning is possible. (Adjusting from a weekday to a weekend might also work.) If your intended airport is full up, perhaps another airport will work. You can usually get into nearby Seletar (WSSL) when Changi (WSSS) turns you down. Another option is to ask for just enough time to drop or pick up your passengers while parking elsewhere.
This varies by location, sometimes you need only know that you have it and no confirmation number will be given. In some cases, you will be given a confirmation number and be required to file it in Block 18 of the flight plan.
Basic CFMU Handbook, Eurocontrol CFMU, Edition No. 14.0, 18 Mar 2010
Eurocontrol CFMU, https://www.eurocontrol.int