1: Get skins from 4 grapefruits , organic if possible, and clean to remove all traces of wax or other products from the skin, then rinse. You should have 4 equal sized pieces of peel per grapefruit.
2: Cut each piece of peel into strips about 0.5 cm or ¼ inch wide.
3: If you keep some flesh of fruits with skins because you have peel fruit completely, don't remove it, actually it's better.
4: Put these strips in a pan, cover with cold water and leave overnight. Next day, change water and leave about 4 hours longer. Note: We do that because we want to remove excessive bitter of skins.
5: After this time, drain peel strips and weigh them. Then put in a pan with their weight in sugar, and their weight in water (if possible filtered or mineral to avoid the chlorine taste).
6: Cook slowly over gentle heat, stiring from time to time...
7: ... until there is hardly any liquid left.
8: Spread out on a cooking parchment on a baking sheet. Leave overnight or longer, until they become a bit firm (to your taste).
9: Then roll in a little caster sugar with a fork to coat them lightly. Keep in an airtight container.
Remarks
It's possible to use other citrus fruit: oranges, lemons, limes and clementines.