Firstly, clean the swimming pool using the vacuum cleaner with the filtration set to "drain". This is the right time to reduce the water level to around 10 cm below the  pool surround, then brush the walls and clean the water line with hth Borkler gel.

You should also remember to remove the accessories as they will keep better when stored dry over winter.

Clean the sand filter by backwashing it for a few minutes, in other words reversing the flow of water through the filte. This unclogs the filter medium, enabling the waste contained in the sand to be expelled into the wastewater collection system.
Rinse for a few minutes; when the sand resettles the waste still present in the circuit is expelled before filtration is once again operated in filter mode.
Now is the time to stop filtration and use hth FILTERWASH.
For a filter of up to 50 cm in diameter it is recommended that you pour 1 to 2 litres of hth FILTERWASH either directly into the filter when it is off (the most effective solution) or, if the filter cannot be opened up, pour in hth FILTERWASH upstream of the filter for a few seconds (into the skimmer, pre-filter etc.) while the filter is running, then stop filtration. In both cases leave the product to work for between 12 and 24 hours. If this time is exceeded you will have to backwash and rinse the filter again copiously. Filtration can then be switched on again.

Prepare and treat the water, adjusting the pH to between 7.0 and 7.4.
With your filter running continuously, it is recommended that you deliver a shock treatment by pouring 300 g of hth Shock per 20 m³ of water directly into the pool, in front of the return fittings.
48 hours after the shock treatment, lower the water level to 5 to 7 cm below the return fittings and drain the water out of the pipes, pump and filter.
To prevent the formation of lime scale and control the proliferation of algae, pour hth Super WINTERPROTECT directly into the pool while walking around it, adding 1 litre per 35 m³.

Lastly if you live in a region subject to harsh winters, protect the swimming pool from frost by installing a line of water tubes diagonally across the pool and placing a gizzmo in each skimmer, after having removed the basket.
Next, place a rubber plug in each return and each vacuum brush attachment and screw it in to replace the fitting. Lastly, disconnect the electrical circuit.

In addition, it is recommended that you install a winterising cover because this protects the water for external material (leaves, dust, dirt, bad weather, insects, etc.) and limits the proliferation of algae by keeping the water in the dark. This will make it easier to start up the swimming pool at the beginning of the season.