To keep things simple the two things you need to focus on are MOISTURE and TIMING.
Moisture – Make sure your soil is damp, not necessarily on the surface but 1cm under. Remember to use mulch which will keep your root zone cool and irrigated, even if there is sun damage your plant will have its best chance of recovering.
Timing – When it comes to watering timing is everything. The best time is at the end of the day on dusk. If you can’t do it at dusk then do your watering at dawn before the day begins prior to 8am.
Points to remember when watering;
– You must get to the root zone.
– Avoid watering plant leaves because that can burn and blister the leaves.
If you have ferns or sun tender plants if possible erect a shade cloth.
For potted plants move them into the shade or even inside on those extremely hot days.