1 - Earn more
2 - Spend less
3 - Save the difference
That sounds very trite, but that's exactly what you need to do.
Earn more by: selling some of your stuff, taking surveys, getting a new/extra job.
Spend less by: cutting out discretionary spend (coffees, magazines, subscriptions, anything you want but don't need), shop around when you need to buy necessities, switch providers for electricity/gas/insurances/broadband etc., cut down on treats, drive or take public transport less and walk/cycle more).
Save the difference: use round ups if your bank has them, save a set amount in a regular saver account which gives high interest.
The effort you put into all those things will determine how quickly you can get to your £5k target.