Thoroughly mix the rice vinegar with salt and sugar while you stir it all in.
Best to use a saucepan on low heat.
Then, slowly add the lemon juice to the mix while stirring.
Keep heating the mixture until the salt and sugar completely dissolve. It'll be immediately evident when this is because you won't see any granules. This will normally take around 15 minutes or so.
Remember to keep it at a low flame, allowing sufficient mixing. But make sure the mixture never boils at all.
Once a homogeneous solution is obtained, take it off from the stove and allow it to cool before using it on your sushi.
For longer shelf life, you can keep it in the fridge.