I-90 and Blewett Pass is the fastest and easiest way to get to Leavenworth.   Plan on a 3-3^1/2 hour drive.  I-90 is 2 lanes each direction with a 70 mph speed limit.  97 is a country 2 lane highway with occasional passing lanes.  There are several trucks.

From Spokane, get on I-90 heading West to Ellensburg.  After you pass the turnoff to Yakima, watch for the signs to Wenatchee.  When you take this exit, follow it to the right.  At the stop sign, turn left.  This is a country road that will take you to Highway 97.  Follow the signs to Wenatchee.  You will be heading North on 97.  After about 10 miles you will come to a T where you turn right.  It is clearly marked with another Wenatchee sign.  You will cross over Blewett Pass and descend to the Wenatchee River and Highway 2.  Turn Left on Highway 2 and follow it 5 miles to Leavenworth.