So, after having a couple of questions regarding custom challenges today, it has become apparent that not everybody is familiar with them yet. Nor do many people know how to set them up, sooo, here is how you do it.
Firstly, you will need to log into http://www.halo.xbox.com using the email address that your gamertag was created with. This will take you to Halo Waypoint on the PC.
Once logged in you will be directed to the homepage and at the top you will notice your gamertag with your rank medal next to it.
Now, you are going to click the arrow next to your gamertag for a drop down menu where you will be given a list of options. The one you want to click is labelled 'Challenges'. This will provide a list of the daily and weekly challenges at the top and as you scroll down, you will find a section where you can create new custom challenges (Up to 5 at a time).
Once you have created your custom challenge (It's easy enough to figure out how from there) you can check back to see how you are doing and how many more kills, medals, headshots or whatever your target is that you need.
When you have completed your custom challenge, you must not forget to go back to this page and turn it in by clicking on the challenge then clicking on the 'Turn in' button underneath it.
If you don't wish to complete a challenge for whatever reason, you may either forfeit it or just leave it to expire. Just remember you can only have up to five custom challenges at any one point. If you wish to make new ones, you must either forfeit or complete a prior challenge.
So, have fun earning those credits and ranking up people! Also, if you wish to invite friends to your custom challenges, the option is supplied. Just remember to message them and let them know as they too will need to log into their waypoint and check their invites there!