So, I just did a check on this (using the current time), and here's what I got:
- Take the Yellow line to the City Hall/SW 5th & Jefferson stop.
- Walk to the area of SW Columbia that's between 5th and 4th.
- Board the 96 Bus, going to the Tualitin Commerce Circle.
- After about 26 minutes, get off at SW Martinazzi & Seneca.
- Walk North to SW Boones Ferry Road and take a left and take a right onto SW Tualitin Road.
- Go straight to the park, which will be on the right.
There seems to be different options for Saturdays, arriving by 8am, but an alert regarding the last route ending after this Sunday. For future reference, whether going to google maps or Tri-met, the stop ID that you want is 3779.