Uk Spouse visa application

[sunlight14]

Hi
I am going to be submitting my documents for my husband in Pakistan to apply for a spouse visa (I am in the UK).
Can anyone advise on what a covering letter should contain and how long it should be. My husband is a second cousin and I have known him for many years as a cousin prior to getting married. Should I mention this in my covering letter ie that I have known him many years before getting married too.

Also in the application it states that a date of travel should be written but surely that depends on when he gets he visa-what is supposed to be written in that box?

Any help would be appreciated especially with the cover letter