Whats the most practical way to keep the temperature up when changing water also how much is too much of a water temp swing. Everything I find online about water changes arent very descriptive. 

When the water in the bucket or coming out of the hose feels about the same as the tank it has always been OK  with me.

Wouldn't worry about it.  For example if your tank is running at 25C and the tap water is 20C then a 50% water change would only lower the tank by a maximum of 2.5C. If its during the winter and there is a very large difference in temperature between tank and tap then you could just do smaller water changes more often - say 25%.  You could also just add some water from the hot water tap but wouldn't worry about it at this time of year.

 

I try to keep it less than plus or minus 2 degrees.  Easy enough to do when filling from the kitchen sink with a hose.

All you need to do is add enough hot water to the bucket so it feels similar to the tank water. Also depends on the fish as some are more sensitive to temp changes than others. My African tank used to get 50% water changes by hauling the garden hose through the window and topping up with cold. The fish would swim in the stream of cold water and loved it.

When changing the community tank in the lounge, I do not want to pour cold water over my hand so have it just warm enough to take the edge off. No idea what temp it is compared to the tank but never had problems.