I had to do this myself in the UK; it is a time consuming process....

Basically the steps are:

1. Apply for police clearance certificate from local police station. (takes around 3-6 weeks...)
2. Go to the British Foreign and Commonwealth Office legalisation office (located near trafalgar sq. in London - search the FCO website to get location and opening hours + be warned it gets busy). It gets stamped with a seal.
3. Take the form to the Qatar Embassy in London. (Takes about a week). I believe you can get this done in Qatar at the Qatari Ministry of Foreign Affairs but you 100% need it stamped by the FCO in London before this... Suggest you get it done in London before you leave.

You will need to have done this before you will be granted your Residancy Visa in Qatar which you need before you can do most things... In other words it is a requirement (along with passing your medical, doing fingerprinting etc.) and not having it done will prevent you getting your residancy visa until you have obtained it - you will still be able to enter the country on a business visa however in the first place but you need the residancy visa for you to officially become an employee of the company.

Best of luck and welcome to Qatar in advance!