Know what you’re going out to take pics of. If you just build a hide at random there’s a good chance you’ll see nothing at all.
You can get some good shots just set up indoors trained on a selection of birdfeeders (ideally arranged to look natural if you want to fake things a bit), same with attracting foxes, hedgehogs, badgers etc into the garden. My Mum’s garden is regularly visited by semi-tame urban foxes, much easier getting shots of them there than out in the wild!
Or as ampthill says, go out to somewhere there will definitely be wildlife. Bird of prey centres, wildfowl reserves etc. 300mm is still quite short for true wildlife stuff, most people seem to go for 500mm+ so I’d be going somewhere you know you can get close.