Hiya Allison, "Yeast starters" are typically used by brewers rather than wine makers perhaps because dry yeast does not really need a starter in quite the same way that liquid yeast does. A starter is when you pitch the yeast into a small volume of fermentable solution (in this case, a cup or so is perfectly adequate but where the SG of the sugar solution is not going to exceed about 1.040. In other words, you want to grow the colony of yeast. Here, you might simply use a cup of apple juice as long as you know that the juice has absolutely no preservatives (especially no sorbates). Follow any/all yeast rehydration protocols and pitch the rehydrated yeast onto the fruit juice - and that juice + yeast becomes your starter. What you want to do is agitate the juice + yeast to keep it aerated. This aeration encourages the yeast to focus on budding (reproducing) rather than pumping out ethanol. What you are looking for is a relatively large, active, and very healthy colony of yeast that will be able to neutralize any systemic problems in your stalled batch as long as you don't swamp the starter with too large a volume of problematic sugar solution - which is why you "feed" the starter by doubling its volume once you know it has no problem fermenting what byou lst fed it.