Like anything it is easy if you know but some don't so here we go!
Firstly download Ghosti's
Resigner and
xplorer360 Extract both of these to a folder of your choice using
winrar (I personally put things on my desktop) To read more about this process go
here.
The program can only understand xbox 360 con files. Files from xploder (.gsx) or rar files will not open! If you have a rar file make sure you have extracted it first.
Before we start you will need to know your device ID's so that we can give the save you want to use the correct info for it to to be read on your 360. To do that we are going to use xplorer360 to extract a save you made.
Double click the xplorer360 icon

and then Click Drive/open/hard drive or memory card
Once in double click partition 1 if using a MU or Partition 3 for HD, it will then show you the devices contents. Profiles are on the left and start with an E.
When you click a profile the right column will show you the games. You can easily decipher which profile is yours by clicking the game folders and seeing the names of the saves. The game folders generally start with a 4 or a 5 and within it will be a folder 00000001 which contains the actual save.
When you are happy that you have a found a save that you made drag it from the profile to your desktop (you can then move it if you want)
Now you need to open Ghosti's resigner, double click the xploder resigner icon
To get in you just need to enter your details for this site.
Click open and brose to the location of the save you took from xplorer360
It will then display your Profile, Device and console ID's. To make the whole thing easier for the future you can tick the 3 boxes and press save ID's that way it will remember your details for the next time you want to resign a save. (Just like xploder does)
Now you have your ID's saved press open again and load the save that you want resigning. All you have to do is press load ID's and resign.
A box will pop up that says resigned and that is the whole process done.
All you have to do now is put the save back onto your profile using xplorer360, I have wrote a guide on xplorer
here but in taking off the save to begin with you should already be familiar with how it work just remember it has to be in the right folder so you can save your game first and then replace your save with the resigned one.
As with all my guides I consider them completely idiot proof, so if it looks long winded it is because I have explaind it in detail. The whole process takes less than a minute. I hope this helps - Gonzo!