I have pretty extensive experience blending wine in my winery over 15 years. There are two types of blends. Pre-fermentation and post-fermentation. The pre-fermentation is usually called a "field blend". This was the old style way of blending wines before we started doing it in the lab. Growers would plant a variety of vines, for example Cabernet Franc, Cabernet Sauvignon and Merlot in Bordeaux, to hedge their bets on what kind of year it was going to be since they all ripen at different times. They do bring different flavor components to the blend. In Ye Olde days, they would just throw everything in the fermenter together relying on mother nature to give them a good blend.
Today, most blending is done post-fermentation. Very few people do field blends because they want the control over the blend. This is usually done in a laboratory setting. You would ideally want a few beakers with accurate measurements on it. Taste the two individual wines. Then I would start with a 50/50 blend and go from there. If you like more Riesling, then start blending a 60/40 blend of Riesling and Gewurz. Keep doing this until you end up with a blend you like. I usually will create a series of blends and keep them in separate glasses so I can go back and forth smelling and tasting them and keeping notes as I go.
You might dump some wine down the drain to get the blend you want, but once you figure it out, you can go straight to make that blend in the future. I would get an empty wine bottle and put your blend in there. You will have 2-3 days to drink it before it starts to go bad unless you make special effort to preserve it. In theory, you could by X number bottles of Riesling and Y bottles of Gewurz and make a blend of several bottles and rebottle it.
Here is a good article on how you can do it at home
