It's pretty simple to use Regent's Canal, although it's slow and getting to it from Paddington is counter-intuitive...
http://cycle.travel/map/journey/11852 is the first bit of the route - just stay on it instead of climbing up at Royal College Street. It all gets a bit annoying around Camden Lock Market: cobbled bridges, dismounts, loads of people to push through. Oh and you get diverted off in Islington when the canal goes in a tunnel.
The alternative I use more often is to go out the front of Paddington, up the old taxi ramp road, then almost straight over into London Street, then a sort of 10/11 o'clock turn by some cycle parking one junction before the end (Strathearn Place, says the map), then straight on as much as you can into Connaught Street, left fork at the Duke of Kendal into Kendal Street and keep straight-on-ish (riding around squares and across staggered crossroads - you get turned right down Spanish Place at one point, but just take the next right into Hinde Street) until there's no street ahead at Wimpole Street, turn left (it's one way) and ride north until you can turn right into Weymouth Street and that puts you on a cycle route signposted towards King's Cross (protected in places, which I quite like but many on here say they don't) and if you keep going straight on to the end (not turning off for King's Cross at Regent Square) and then follow signs (turn right first at the end, I think - apparently this is London Cycle Network Route 0). Turn left at the end of Lever Street to cross City Road and enter Hackney... but whether that's useful depends where in Hackney you're wanting to go

I do have a sat nav app, but I struggle to hear its voice prompts over the traffic noise and I don't really like stopping to check it every few minutes, so I tend to follow the signs unless they look like they're doing something silly.
http://cycle.travel/map will give sensible routes that print nicely, but it sometimes struggles with London's unusual cyclescape.