I don't know exactly what you have for a setup, but here is how I do it.
Using a 5 gallon paint strainer bag you can mash like you would traditionally, except in a 7.5 gallon pot.
Then, after the mash, if you have a 5 gallon pot (such as from stovetop brewing days), you can take the bag and put it in the 5 gallon pot and pour sparge water over the grains. When the pot is almost full, stir the grain and let it sit for a minute. Lift the grain bag out and dump the sparge water into your 7.5 gallon tank. Repeat the sparging until you reach your desired starting boil volume.
Hopefully this makes sense. I've just gotten my first few 7.5 gallon brews fermenting in the last couple months but have had good results with this method. I am still dialing in my updated system but get about 70-75% efficiency even with the so-so crush from my LHBS.