Tipping on Kilimanjaro is a bigger deal than tipping anywhere else on a Tanzanian trip, because the crew is bigger than anywhere else. It isn't compulsory, and it is genuinely expected.
How the pool works
You don't tip each person separately. The group puts together one pool at the end, and it's divided by role: the lead guide takes the largest share, then the assistant guides, then the cook, then the porters, who are the largest number of people and the smallest individual share. Your guide will usually suggest the split, and a good operator gives you the breakdown before you fly.
What the rates look like
The guideline figures in common use are per day and come from the whole group, not from each climber. Roughly: 20 to 25 US dollars a day for the lead guide, 15 to 20 for an assistant guide, 12 to 15 for the cook, and 8 to 10 for each porter. Multiply that by your crew and your days and you'll see why people get caught out. On a seven-day climb for two, the pool runs to several hundred dollars.
Those are customs rather than rules, and they do drift, so ask your operator for the current breakdown for your actual crew size instead of budgeting from a website.
Bring it in cash, and bring the right cash
US dollars in notes, and notes printed in 2009 or later. Older ones are widely refused in Tanzania and you can't fix that at 4,000 metres. Bring a mix of denominations rather than a few large notes, because the pool gets divided a dozen ways. There are ATMs in Moshi and Arusha and none on the mountain.
When it happens
At the last camp or at the gate on the final day, usually with the whole crew gathered, often with singing. Someone from your group says a few words. It's a proper moment rather than an awkward transaction, and it's worth having the envelopes ready before it starts.
If something went wrong
Tip less, and tell the operator why. A tip is a thank-you, not an invoice, and we'd much rather hear that something went wrong than have you quietly pay for it anyway.
One thing worth doing
Hand the pool over in front of the crew rather than privately to the lead guide, and say what the total is. It's normal, nobody finds it rude, and it makes quiet skimming much harder. For how tipping works on the safari side of a trip, see tipping in Tanzania.
If you're climbing and going on safari
They're separate pools and separate people, so budget them separately. The climb crew are tipped at the gate on your last day, and your safari guide at the end of the safari. Don't let one eat the other, and bring enough cash for both, because there are no ATMs in the parks or on the mountain.