score:9
Buy 4 metro tickets (or if you are going to do the same thing on the return trip, you are probably better off buying a « carnet » of 10 tickets — note that those used to be available at the Eurostar bar, not sure if this is still the case).
Follow the signs for RER D towards Corbeil or Melun. It’s usually track 44, 3 levels down from Eurostar arrivals.
Take the RER D to Gare de Lyon (2 stops, less than 10 minutes).
In the best of conditions it takes less than 15 minutes. I would count about 30-45 minutes in your case, especially if you have to buy the tickets and you get in Paris early in the morning and it’s the beginning of a month (lots of people renewing their monthly passes). If you have the tickets already it should really be easy.
Check for any disruptions in advance (there are lots of works during the summer).
Note that if there are issues with RER D, there are alternatives: