Pacun, get on your state website where your client is. Might not be DC, but possibly MD or VA. There should be an IFTA reporting website online. You will need for each unit, miles driven in every state, and gallons purchased in every state. If there are many units, this could take some time.
Miles per gallon will be calculated by the website for each unit (truck). Let the website do this, don't depend on the client to tell you how wonderful his mileage is. The website should divide the mileage by the MPG, to derive the gallons which should have been burned in each state. If you have purchased that many gallons or more in that state, you will be entitled to a credit. Credit is calculated by the fuel tax per gallon times the "excess" gallons.
The reverse is true if the unit has not purchased enough gallons in a state where they have burned gallons. The IFTA charges you for those states. The net of all the charges and credits results in an excess charge that you have to pay, and if there is a net credit, you simply apply it to the next quarter.
Some states (e.g. Indiana and Kentucky) have a surcharge that is not charged at the pump. The surcharge is built into the IFTA calculation such that if your truck travels through such a state, you are likely to have to pay extra.
Drivers love to fill up in South Carolina, who has the lowest pump tax of any other state. They will drive across the line from Charlotte, NC and places in Georgia just to buy cheaper fuel. However, if they don't drive more than this in South Carolina, the calculation will result in a credit from SC and a whopping charge for GA or NC. It all works out.