Unlike Carnegie Hall, the answer isn't just "practice". For more than one reader, the promised five minute walk from New Street station to the appallingly under-signposted Symphony Hall has turned into a twenty minute ramble through interminable shopping arcades and deserted streets.
The CBSO seem unwilling to share the innermost secrets of their dominion on their website, so with Tristan und Isolde and Jonas Kaufmann coming up shortly, here is the secret route that will get you there in a jiffy, plus a dining warning.
The key is to leave New Street by the Victoria Square exit, not the main station exit. If you remember nothing else, remember that bit. It's clearly signposted, towards the front of the platform if you're coming from the south, and it brings you out quickly and simply to Navigation Street. Turn left, and you're almost done. You then have a choice of two routes.
The first, shown in blue on the map below, is to turn right into Hill Street and walk uphill towards Paradise Place, a cluster of fast food outlets. Then turn left towards Symphony Hall. This whole area is pedestrianised and can be confusing, especially in the dark, so the alternative below may be easier.
For the second route, shown in red on the map below, turn left on to Navigation Street as before. There's a small Sainsburys on the left which stays open late, handy for drinks and sandwiches seeing as the Victoria Square exit bypasses the delights of station catering. Continue beneath the flyover, following signposts to The Mailbox and rainbow lines on the pavement. As soon as you're through the underpass, turn right and continue (hotels on your left) to Broad Street; Symphony Hall is then just across the road on the left.
The cheapest trains from London Euston to New Street are £12 return (London Midland) or £15 return (Virgin). It's even cheaper (£10 return) to go from Marylebone to Birmingham Snow Hill, and not much further to walk, but the last train leaves a bit too early for most concerts (21.42 weekdays, 21.20 Sat, 21.15 Sun). Last trains from New Street to London are 23:10 weekdays, 21.53 Sat and 23.00 Sun. This makes it worth checking the concert end time if you're going on a Saturday - Tristan is supposed to end at 21.30, which is fine, but Opera North's Die Walküre later in the year won't finish until after the last train has left.
If you're driving, follow the signs to the NIA/National Indoor Arena, which is five minutes walk from Symphony Hall and has its own large car park (map). There are other car parks in the area if that's full, but they all fill up quickly when there's anything on at the NIA, so you may end up parking some distance away. The most reliable area for free street parking in this case is south of New Street station, which is a bit of a walk.
Theoretically there is food at Symphony Hall, but it's pretty grim and most times I've been there it has run out (totally) before the concert interval. The alternatives are to pick up something en route as mentioned or to dine in the Brindley Place area. This is reached by walking straight through the main entrance of Symphony Hall then out at the back. You will immediately see a number of chain restaurants, the most reliable of which is The Handmade Burger Co. All of these get busy, especially during concerts, and any which take reservations are probably by now full for the 1 hour interval in Tristan. So unless you want to fall back on Greggs in Paradise Place, a BYO picnic is strongly recommended.