Make soap into tiny flakes with knife.
Set in pot half water to boil. Using a whisk dissolve flakes well into water (hobs still on).
Switch off hob, then add the soda and mix gently. don't let it foam!
Let cool a little, but not too long
Transfer the mix to the bottle with a funnel.
Warm remainingwater at about 50-70°C. Rinse the dirty pot to , then tansfer also into the big bottle.
Do NOT shake the bottle, otherwise it will foam and not homogenize.
Turn the bottle quickly upside down, let it stand for some time then quickly turn again. Repeat at regular intervals.
You should see the 2 phases at first separated, but then getting perfectly mixed together.
When it looks homogeneous you can let it sit.
If instead the 2 phases do not integrate: pour the detergent back into the pot and turn on the stove until completely transparent, then let cool slightly, then re-bottle. Perform again the inverts.