Wipe the scope tube off with rubbing alcohol to get any dirt and oil off.
I took the scope covers off and painted them separately inside and out (the visible parts). Once they dried, I used them to cover the lenses and painted the whole scope. It leaves the front and back portions of the scope without paint, but I'm never going to run the scope without covers so it doesn't really bother me. Come to think of it I painted it with the rings on too.
If you want to do it without covers then use tape. The best would be blue painter's tape, but almost any tape will do. Put the tape in a cylinder shape like a sunshade, and have the sticky side facing out. Then cover the open ends with tape. Take your time and be careful! It would suck to accidentally bump the tape and get paint on the glass. yikes!
For the turrets I ran one piece of tape around the portion with the marks on it. On the side of the caps where it says which way the adjustments go, I used a compass and a razor to cut tape into a donut shape. It was pretty good, but not perfect. Anything else you don't want to get covered in paint, put tape over it. The more time you take in getting everything ready the better it will look in the end.
Mine wasn't sponge camo, but the concept is similar. Lay down a base coat of the main color you want, in my case dark green. I pulled some grass and leaves out of the yard and sprinkled them on, painted certain spots in tan, then kept adding layers in light green and dark brown. I'm pretty pleased with it. The picture doesn't really show the fine details.