I often hook in first and then get into the straps cause hooking in as quickly as possible means my arm/shoulder muscles get tired less quickly, especially in big winds

. But I reckon whether it's best to hook in first or get into the footstraps first depends on the actual situation.
In marginal conditions on a bigger board I actually pump, get in the front strap, hook in and then get in back strap.
For a beginner on flat water I'd recommend getting into the harness first, cause it makes it easier to create mast foot pressure and get into the straps, and it saves energy. If the water is very rough or there are serious control/catapult issues then I'd agree with the previous posters that it's better to do footstraps first.