Swamp: light and moon phase
For regular Java 1.20–1.21, look for slime at night in the swamp at altitudes of approximately Y=51–69. Illumination should be low; the full moon is more profitable, the swamp spawn does not work on the new moon. Do not flood the entire area with torches before searching. Reference: Minecraft: slime.
Walk around dry areas, keeping a safe distance from the skeletons. After the large slime, medium ones appear, then small ones; the balls fall out at the last stage. One killed large slime is not yet collected prey.
Underground chunk: confirmation first
Underground spawn comes in suitable slime chunk below Y=40 and does not require darkness. chunk has size 16×16. In Java, boundaries are visible through F3+G; consider negative coordinates along the boundaries of the chunk, and not by rounding to zero.
Make a floor in the selected chunk with a reserve of 3 free blocks on top and light it from regular mobs. To test Java, move further than 24 blocks, leaving the pad in the active area. Cave mobs around can take up the total spawn limit.
Before copying the module, perform counting tests: Testing a farm before building it at scale.
Don't build a collection before the mob appears
The calculator needs the correct seed and edition. Java and Bedrock use different rules for defining a slime chunk; the result of someone else's settings is not transferred. If the seed is not available, mark the real underground meeting places and check with the site.
Once confirmed, collect loot manually several times. Only then add bait, kill and hopper: this way an empty chest does not confuse a spawn error with a transport error.
To connect the receiver to the chest, the following instructions will be useful: Hoppers: send items to the right container.