If you have some questions about how to make a claim, then this is the place for the FAQ answers.
Needless to say, the FAQ answers have to be general in nature and you should take personalised tax advice on your own circumstances.

When can I claim?

You must have a qualifying period of 365 days before you can make a claim this means that there is a period of at least one year in which the first and last day (or later) are spent working abroad.

There are some other tests that have to be passed.

You do not have to be outside the UK on the 365th day, but you must be outside the UK on or after that date.

Explain the day count?

You must spend more than half your awwwtime in the 365 day period outside the UK.
If you are outside the UK from 1 January to 30 June and then 1 December to 31 January, then you will qualify - subject to the other conditions

If you are outside the U.K. 1 January to 30 December only then you will not qualify, as there is not a 365 day period.

In the above example, if you went abroad again for work on 30 January in the next year - even for a week - then you will qualify.

As simple as that to count the days?

Not quite. Inevitably.

The check on the half day count has to be undertaken every time you leave the UK to make sure you don't spend too much time at home. If you spend 30 days at sea, miss your next trip and spend 40 days at home, then you have failed the test as you have spent 40/70 days at home, which is more than 50%.

That's where holidays come into their own.  Hopefully these FAQ will help you

If you missed the trip above, but went to Tenerife for a fortnight, then you have spent 26/70 days at home, and passed that test.

So all my income qualifies?

Of course it doesn't. The only income that is tax free is qualifying income, which is broadly that income earned for performing the duties of a seafarer and paid to you as an employee.

So if you are abroad for 365 days in a row employed as a seafarer, and have a qualifying period, but 25% of that time you are training cadets in a classroom in Singapore, then only 75% of your income will be tax free.

Do I have to be at sea to qualify?

No you do not.
If you spend your entire trip abroad in dry dock it will still qualify as long as the hull is not pierced, so that the ship remains physically able to go to sea at all times without sinking!