First off, sorry the strumming approach for this song is a little confusing.

Click on the Notation tab, and in the first bar you'll see the downstroke/upstrokes for each strum.

Next, slow down the performance video to half speed (click on the cogwheel on the bottom right of the video panel, and select SPEED - 0.5x), watch the first 2 bars repeatedly, and match your strums to the notation.

Slowing it down should reveal how the strumming pattern should go. Essentially, you are using the downstrokes as steady palm-muted 8th notes, with some accented upstrokes in between (16th note accents).

Let me know how it goes

Mike