Splurge, and buy yourself one of the "4 week" Riesling kits. For a
similar style, with a little more sweetness, try a Piesporter. The
white kits (IMHO) turn out pretty well, considering how inexpensive
they are. Around $1.50/bottle for the ones I buy. You can make
drinkable wine using grape concentrate from the grocery store, but I
doubt you're going to come very close to the taste of a Riesling using
those kinds of ingredients. If you're determined to do it the hard
way, you could try adding some dried elderflowers to the initial
fermentation - they are typically used to impart some floral notes to
the Piesporter kits.

Doug