Don't do it, they are rubbish
I don't agree. I have used the Raynox 250 for a year or so and it resulted in some good shots. The quality is a lot better than the all the cheap +something filtrs.
Stick to the Sigma 70-300mm, I've actually used the same lens. 50mm is too short, the magnification isn't big and you'll most likely get the filter holder in your frame. You should start at about 70mm. As far as I remember, you get 1:1 magnification at about 100mm when the lens is set to the minimal focusing distance. The fun really starts at 130-150mm. At 200mm you get about 2:1 magnification, and at 300mm with the sigma set to macro it's an outrageous 4:1 or so. Of course on 300mm it's very hard to get a good shot, even if you get the focus spot on, the sigma isn't exactly a performer at the telephoto end.
Be sure to set your sigma to manual focusing. The AF will struggle with the macro lens on. Focus by moving your camera closer of further from the subject. It will be hard to get a sharp photo at first, but don't give up. After a few hundreds of shots you'll be getting quite a nice percentage of sharp photos. It's just the way you work with a macro converter, you have to check your LCD, magnify the preview and see if you got this one sharp. Start using the lens at the shorter focal lengths, then move up. You'll probably stick to 70-150mm. Beyond that it seems fun at first, but it's usually just too much, not to mention difficult.
I really loved using the Raynox 250, it's a lot of fun. If you can't afford a proper macro lens, there's no cheaper way to shoot macro.
Here are a couple photos of mine, shot with a Canon 400D, Sigma 70-300mm and Raynox 250.
These are even older, shot with a Panasonic FZ7 P&S and the Raynox 250.