The simplest form is one word per trade chosen from a short, fixed list, for example calm, focused, confident, anxious, frustrated or elated. A fixed list prevents the log from filling with vague entries such as fine, and keeps the act to a few seconds.
Naming a feeling has two uses. In the moment, putting a specific word to an emotion is associated in laboratory research with a reduction in its intensity, although that work was not done on traders. Afterwards, the words become data: you can group trades by state and compare the average result in R of each group.
Honest limits apply. People under-report states they dislike, a word written after seeing a loss may describe the loss more than the state beforehand, and a state and a bad trade can share a cause. The log shows where to look, not why.
Record the word before analysing the trade or looking at the day's total. Even then, treat early patterns as questions until the groups have enough trades to mean something.
Used steadily, an emotional log often reveals that a small number of states account for most of the costly trades.