It's all a bit 'rule of thumb' Lori!
I put two pints of water in a pan and add about four or five handfuls of washed mint. There's no need to remove leaves or flowers, just put the whole lot in. Bring slowly to the boil and then switch off the heat and leave it to cool. Then strain it through fine muslin (or a paper towel!) into a glass demijohn or whatever vessel you'll use to ferment it.
Dissolve 3lbs sugar in another two pints or so of water and, stirring, bring that to the boil. Let it cool and add it to the mint liquid in the demijohn. Add three teaspoons of lemon juice, then top it up with plain cold water. Leave it overnight, then add some dried yeast - it works with either wine or bread yeast - about a teaspoonful. Put a fermentation lock in the jar. My grandmother used to put a balloon over the top, but I think a lock's better!
Let it work until the sediment drops - that'll take a week or ten days depending on the temperature, then rack it off into a clean demijohn. Now leave it for another two or three weeks and rack it again. I add a crushed Campden tablet at that point and leave it overnight before bottling. Be aware that it might still be ever so slightly fizzy and try to push the corks out!