Continental route is the most crucial as with exception of Scotland, no-one will be going after the nordic regions within the first 10 turns.

Try to split your forces as much as feasible to take several towns at once, laying extended siege when your troops are not sufficient for an assault. Recruit as much cavalry as you can early on because if you can lay siege to a town and it sallies, 2 units of cavalry can break just about any rebel force streaming out of the gates by charging from both flanks.

Make use of mercenaries (mainly crossbowmen for open field battles and besieging villages or spearmen for capturing walled towns)

Also, to finance your early expansion, set all tax rates where possible to very high and do not convert settlements as this will slow your start down.

The first 10-15 turns determine the future success of your empire - so gain regions and finance your strategy by selling alliance+maps+trade for ~ 4000-8000 florins, depending on faction and diplomat skill. Make sure to sell it for tribute over turns (no more than 1500 a turn or they will not accept more often than not)