Copy paste this on your address bar: "3ds.guide" That's the most comprehensive and noob-friendly step to step guide to installing CFW! However, first, check to see what CFW your old 3ds is running exactly (eg: Luma). If you simply have an old Luma, you can also freely update the CFW then it'll be ok to update the 3DS to 11.6 and keep updating normally without losing any functions or risking anything.