Not that much too it. Drop a couple of anchors tie them to the slider!
It helps to have the lines tight so that the slider is held stationary, but it isn't essential. It isn't the ropes that hold the slider stable when you hit it, the slider has to be self stable.
If you want the slider to remain in position then you have to anchor it in relatively shallow water. With wind, waves, current, etc. the slider will likely move about as much as the water is deep, even with the ropes tight. If there is slack in the ropes the slider will move even more
If the water level changes then you will obviously have a problem with leaving the slider in place. Reservoirs, by there very nature, change water level a lot over a season but very little over the course of a day. Tidal waters, such as the Delta, will change water level a few feet twice a day.
For best anchoring you will want a "scope" of at least 5:1. That means that if the water is 20 feet deep, the anchor line should be 100 feet long. For short term anchoring you can often get by with 3:1, or 60 feet of rope for 20 feet of water depth.
Choosing the right anchor for the bottom is also important. For a muddy bottom mushroom anchors work well. A small danforth works in sand, and grappnel anchors are sometimes useful for rocky bottoms.
If you intend to leave the slider in place for long periods and need to deal with changing water levels, then start with a lot of scope (5:1). Instead of tying the line to the slider at both ends, tie one and and on the other pass the anchor line through a ring and attach a heavy weight. The weight will pull the anchor line taught and allow for fluctuating water levels.
As I said, it isn't all that critical that the slider be held tight. I use a rave inflatable slider that I put out on the delta when we take the houseboat out. Because of the tides I can't keep the lines tight and I don't feel like messing with self adjusting systems. The wind usually keeps the slider pushed to one end of the slack and it stays relatively still.