Crispin, please do not add the salt directly to the tank or sump. You must first mix it up (with RO water) to the desired SG and ideally let is mix with a power head in the bucket for about 24 hours before use. Make sure it matches the tanks temperature before use as well.
Using a thin pipe, (not airline thin, 10 to 12mm should do) siphon out as much detritus and unwanted algae (if you had) from the main tank as possible. The reasion for the thin pipe is that it will give you more time to get around the tank. Also try to remove as much detritus from the sump area as well.
Once you have taken out the same amount as the newly make up water, you can simple pour/siphon the new water into the sump and turn the return pump back on.