From Bellingham, drive south on I-5, take the Cook Road exit and drive east through Sedro-Wooley and out on Highway 20.
Drive through Marblemount. The turnoff for the takeout is several miles past Marblemount at approximately mile 111.5. As you drive down a pleasant straight-stretch through the trees, you will see a distinct road to the left. That’s not it. Directly opposite that road is a small road turning off to the right (just before the Curve sign with the 40 mph warning). Take that and you should see a small sign identifying it at NP-213. Follow that for 1/4 mile and you will come to a fork in the road. Take either fork – they form the loop at the end, complete with a nice vault toilet.
If you miss the turnoff, just around the corner, you will come to the big sign marking the entrance to North Cascades National Park. That makes an easy place to turn around and go back to the road that you missed.