When a couple decides to get a separation (with a view to divorce), what is the first step to take?

While it is not strictly necessary for parties to sign a Deed of Separation to indicate their intention to separate, some parties may decide to do so in order to regulate their financial obligations towards each other during the separation period. This is especially important if they co-own a property together. They may also decide to sign a DOS if they have children and want to be clear about custody, care & control and access issues.

The short answer is that a 'legal' document is not necessary as proof of separation. For example, if u physically live apart, it will be easy to prove. Do take note that a deed of separation is not a court document, it does not guarantee a divorce will be effected at the end, etc. Of course it is an advantage to have an agreement drafted out, it could include other things eg child care and control, alimony, division of assets in event of divorce, etc.

One important question to ask is that will it be a uncontested divorce. Uncontested on reason of divorce, uncontested on assets, etc.

Few will actually have video evidence, it is not necessary to have that. Unless u want proof of say physical abuse. Writing in diary will help yourself to remember the dates/happenings as detailed as possible. With these details will be your case of proof. But depending on what is the situation, circumstances evidence will always be good, eg. recording of scoldings from the spouse.

Remember the courts determine unreasonable behaviour as a series of event, not just a one off incident. And the cummulative effect resulted in u unable to live with your spouse.

If the couple already can go on mutually agreed separation for 3yrs, then don't require to use unreasonable behaviour?
Every couple situation is different. It is best to find a lawyer specialising in matrimonial affairs. Eg, how long has the marriage been into, What is the exact situation. Apart from the legal proceeding part of it to prepare, one might also want to start to look into financial matters first (which typically becomes one of the item of contestion). Eg. Bank account, house, insurance etc. Financial matters will matter into how one will go into the divorce.