I did this a couple of years ago, after having not been in a band for 10 years or so.
Started when my old drummer (who plays in a wedding band too) suggested the possibility of getting back in the saddle.
So I advertised on Gumtree for a guitarist and singer. A guitarist appeared fairly quickly, with tastes very similar to ours, so that was a bonus.
A singer, who I half knew, and who also plays in another local band, offered his services as a singer because he fancied singing without having to also play guitar.
Rehearsed up an hour's worth of stuff over a couple of months, once a week, and played at a local barn dance / mini festival thing.
It then became apparent that the singer liked his beer a little too much, and he didn't turn up for our first "proper" gig, claiming illness. We got a bloke in at short notice to do the gig, which was consequently a minor disaster. We've not been booked back there ...
An ad on Facebook produced a singer who'd done the rounds in lots of local bands. He soon turned out to be unreliable, hence the number of bands he's been in.
So, back to Facebook and Gumtree. Facebook produced a lady singer, who's been with us since. this despite her throwing a strop recently which led to us cancelling a few gigs and looking for someone else. Auditions were utterly horrendous, with no-one suitable turning up. In the meantime, our singer decided she'd actually like to be in the band again. So she is.
I'd think that identifying your market is pretty easy for most pub bands. Are there pubs that put on live music in your area? Are the bands vaguely similar to what you plan to do? Market research complete.
The first serious step is finding others with a similar level of commitment, and a similar vision of what they want to play and how often they want to gig. Never overestimate someone who says they already know the songs you mention to them. They may sing along, or play along, with the record and not have a Scooby how to play them in a band situation.