My log book:

I write down the date and name of the casino.

Then for each session I have two entries: money out and money in.

Typically I start each session at $200.00; if I've any left or if I won I list the remaining amount, otherwise, as usual, I write "zero."

At the end of the trip I subtract money out from money in to see if I've a net profit or loss, then I do the math to calculate the total win or loss for the trip.

That number is added or subtracted from my running total, which I began somewhere around the end of the last century.

FWIW lifetime I am now down about $72K.

While I've never been audited I suspect I could pass inspection.