There is a specific portion on your tax return form, Section 6, on the last page, for this reason.
Then if you read Part 6 (Art 10(1)(0)(ii)) Exemptions, in the information leaflet that comes with your tax return, it states that:
You have to be out of the country for at least 183 full days or more in the year of taxation,
and
for a continuous period of 60 full days within 12 month period.
Then you add an addendum to your return to show the income you regard as tax free.
I have been doing it for several years and have had no problems
.
I also fill in my own form at no cost.