Depth and container decide most of the safety
The two accidents that hurt tortoises at soak time are tipping and drowning, and both are designed out before the water goes in. Choose a low, wide, heavy-based container with smooth sides the tortoise cannot get a climbing grip on: a washing-up bowl or shallow storage tub works better than a bucket, whose depth and narrow base invite flipping. Fill it with lukewarm water, tested with the inside of your wrist the way you would test a baby's bath, and keep the level at roughly the bottom edge of the shell, where the upper and lower shell meet. At that depth the tortoise stands with its head well clear, drinks and soaks its underside, and cannot get into trouble if it rests; deep water that forces it to hold its head up is tiring and dangerous.
Warmth is a room problem as much as a water problem
Chilling happens quietly and usually after the soak rather than in it. Run soaks in a warm room, away from open windows and draughty doorways, and keep the session short enough that the water stays tepid; if it cools noticeably, top it up with a little warm water poured in away from the animal or end the soak there. Never use hot water to compensate for a cold room, because a tortoise lowered into water that feels merely warm to your hand can still be shocked by the contrast. The moment the soak ends, lift the tortoise out, pat it dry with a soft towel — including the legs and the underside of the shell — and put it straight back into its warm enclosure under its normal heating, not onto a cool floor to wander while you tidy up.
Supervision means eyes on, the whole time
A soaking tortoise is never left alone, not even for the length of a kettle boil. Tortoises test the walls of any container, and one that scrambles and tips onto its back in water cannot right itself as it usually can on land; in water, that position is immediately life-threatening. Sit with the soak, keep dogs, cats and small children from jostling the bowl, and if the tortoise is straining repeatedly to climb out, take that as the end of the session rather than a challenge to defeat with higher walls. Struggling means stress, and a stressed soak does the opposite of the gentle hydration and hygiene you were aiming for.
Build the safe version into the routine
Soaks go wrong most often when they are improvised by different people in different ways: one family member's deep bucket is another's shallow tray. Write your setup down once — this container, this depth, this room, this length — and treat it as the household method rather than a personal habit. That is exactly the kind of routine KinsLe's Care Teams is built for: the soak can be assigned as a care duty for the tortoise, each person who takes it on works from the same safe instructions, and the completed soak is marked with a timestamp so you know it happened and how long it has been since the last one.