Let's say a Roman citizen has to pay his taxes. How would he pay them? How would he know how much to pay? How would the tax office make sure he paid the right amount? And if there is a mistake, let's say the citizen mistakenly paid too much or not enough, how would this be sorted out?
Most Roman citizens did not pay taxes, that was mainly for provincials. Although under the Principate we have evidence that tax-exempt status often seems to have been given as a separate grant to new citizens, suggesting that it was not assumed by then, in the Republic and theoretically at least in the Principate citizens did not pay tribute, the most important type of tax, after 167. Roman citizens were liable to pay the portoria, essentially customs duties for moving particular types of goods into particular ports, for rents paid on the ager publicus, and for the inheritance tax once it was established in the Principate. These incomes were paid sometimes in kind but mostly in cash, and even in the Principate were still collected by publicans, though the collection of provincial tribute had mainly passed to the hands of the procurator and his staff. But these incomes were also only the responsibility of a fairly small group of the population, that was engaged in the activities of commerce or pasture on public land. The vast majority of Roman citizens, at least in Italy in the Republic, would have paid no taxes at all
I can try and provide an insight into how taxes within Egypt under the Roman Empire operated, specifically regarding land.
Firstly, documents from the Mendesian papyri^1 suggest that Egypt under Roman rule consisted of at least forty-six different land classifications based upon "the juridicial status of the land (private vs public), its use, and the moistness – and hence expected fertility- of the soil."^2 This itself would suggest that there was a highly complicated procedure in terms of the land taxation system.
On public land (land owned by the state) there was a tax continued from the rule of the Ptolemies, known as the epigraphê under the Ptolemies or tributum soli under the Romans, which was an adjustable harvest tax. The land would be surveyed and information would be taken (size, crop etc.) which would produce the amount needed. Such tax was mostly paid via crop.
On private land, the Romans opted for a low-fixed tax rate which was varied between 3/4 to 2 artabas per aroura (measurements of land).^4 This tax was predominantly paid via crops, but there are instances in which vine and olive yards were paid for via money.^5
Monetary taxes certainly existed within Egypt as well (aside from Alexandria which was mostly exempted). The poll tax (laographia or tributum capitis) was introduced and to be paid by adult males. As mentioned above, there were several taxes upon the trading of goods. Eastern trade (with India and alike) was taxed in the form of tolls, duties, tariffs levied on the goods as well as upon the traders. For example, the tariff of Koptos suggests a guard would pay 10 drachmes, a sailor would pay 5 drachmes, and a courtesan would pay 108 drachmes. We are aware from papyri that Koptos acted as a hub in which goods could be assessed in order for taxes to be charged.^6
The topic of taxation is a complex one and there is much more that could be said, but hopefully this provided a basic insight.
Sources
[1] P. Thumis I (AD 170-171), P. Mendes. Genev (AD 200.), P. Ryl. II.426 (AD 200).
[2] Blouin, K. (2014), Triangular Landscapes: Environment, Society, and the State in the Nile Delta under Roman Rule, Oxford: 139-140.
[3] Rathbone, D. (1993), Egypt, Augustus and Roman Taxation, Cahiers du Centre Gustave Glotz: 84.
[4] P. Lond II 192 (14-37 CE).
[5] Wallace, S.H. (1938), Taxation in Egypt from Augustus to Diocletian, Princeton: 47-76.
[6] OGIS II 674 = IGR I 1183.