Whenever I'd mod my gamerscore I'd mod maybe around 700 or so and do it in the mornings right before I'd get online. So you know how once you start up your Xbox you get to choose what profile to sign in to (well, if you have Auto sign in disabled) well right there don't sign in and go to your Storage and put your profile on your USB and mod it there, so there will be a somewhat long gap from the time you got off till the time you just signed on.

I remember when I modded mine I only did about 500 or so of each game, and I tried my best to put in a realistic date.

As long as you don't go overboard, then it doesn't really matter.

The general consensus is that the smaller you mod, the less chance of being caught but this is untrue. I've seen people mod ONE achievement being reset, it all depends really. I think it does help if you mod smaller amounts rather than large but at the end of the day, you're still risking it.

You should add legitimate looking timestamps to those you unlock. Also set your games/achievements to private, as this will reduce the risk of being reported by other players. Common sense and also not telling whoever will listen online that you've modded will also obviously prevent you being reported and reset.

Even if you do all this, you can still be caught but they will significantly reduce the risk.

You never know if you'll be banned. I had to accounts where i modded all of the games one achievement at a time. I've never been banned and I added Japanese gpds to my account.

On the safe side you might just want to mod one game each a day maybe.