Did you just get regular paving sand or the jointing/ setting type? Regular paving sand can be just brushed into the joints, but will wash away over time and if you already have weeds, won't take them ling to break through again.
If you have the jointing sand, instructions are usually printed on the bag, but clean out as much of the debris from between the gaps using a specialist pointed
brush or a trowel. Once clean and fully dry, you can brush the sand into the joints or pour slowly into the joints. Tapping slabs with a rubber mallet can encourage the sand to settle and fill gaps below. Once you have removed as much of the excess as you can (brush at a diagonal to joints), soak the area before brushing again to remove any excess sand. The setting sand will colour the paving if not fully cleared off, so the quality of the result will largely depend on how little excess you manage to leave behind.