Be aware that all kilt makers use inches, and not centimetres.
   To a European that might be a little bit odd; nevertheless that's the way it is; and recalculation is simple. An inch is 2.54 cm.

1. The length
The crucial question: Can you or can you not use the standard length of 24” or 61 cm?
   Try with a towel, folded to that length and in front of a tall vertical mounted mirror. Or use the self timer on your digital camera, mounted on a tripod at about eye level and at a distance of about 3 metres.
   Chances are you can. If you cannot, find a kilt maker with more options or accept a higher price.

The "correct” length of a kilt is from the middle of the knee cap to 2.5 cm above it. In practise it means a give and take of about 5 cm. The kilt should under no circumstances cover the knee – even if it is rather often seen.
   Traditionally a kilt is sitting higher than trousers (about navel) but a casual kilt does not necessarily have to. Here there is some give and take, too. Due to the wider belt that normally is used with a kilt, it will inevitably sit somewhat higher than trousers, however, about 1 inch.

J. Charles Thompson, almost a legend among kilt wearers, is to the short side. He says in his book "So You're Going to Wear the Kilt" p.99:

“DON’T get your kilt too long! The top of the knee is the absolute long limit, and up to an inch [2.54 cm] shorter is acceptable – better in my personal opinion”.  

It can be expensive or impossible to make a kilt longer or shorter afterwards.

2. Waist size  
Typically it varies by 2 inch-intervals, i.e. 34, 36, 38.
   Be sure to measure the waist where the kilt is to sit. If you are between two sizes, better take the larger of them. The kilt being fastened by means of straps and buckles means some tolerance here, too.

3. Hip size.
If required by the kilt maker take a loose measurement where you are at your widest. 
   Normally this measure is only needed for more expensive kilts.

Difficult?
Not really. You might also just jump right into it, order that inexpensive 24" kilt and see what comes out. Then you'll have some good indications when ordering your next kilt.