Want step by step procedure for data backup from macbook air to external hdd
Hello and welcome to the forum!
I'll just support the suggestions already given: 1) Use Time Machine (TM) on an external HD dedicated for just that purpose (I use two 1 TB HDs for my MBPro & iMac); 2) Use cloning software on another external HD, I have CCC (Carbon Copy Cloner) which gives you a bootable drive (which TM will not) - for myself, both computers are cloned to another external HD w/ two partitions; and 3) Backup your personal files (e.g. Music, Photos, Documents, etc.) to yet another location - again for myself I use an online service for the iMac (i.e. Carbonite) and an external SSD for my MBPro files.
NOW, if you are located where your home and/or office might be subject to damage (e.g. criminals, weather, etc.), then you need to also backup to another location (such as a bank vault, an office, or other site) - my point is that if your 'data' is important (and certainly if critical as part of your job), then redundant backups are recommended. Dave