It all went rather well. We got an English language GPS but the map codes directory were all in Japanese, so the only input option was by use of telephone number. This was a less certain mode as not all telephone numbers are mapped onto the GPS system. But it was quite useful. At the same time I would say that the GPS was not essential. A good map, some common sense, sharp eyes, and a bit of luck will serve as well. Relying on the GPS was the last resort for me.
The best place we visited was Kamiyubetsu Tulip Garden in or around Yubetsu town. ("Kami" may mean "up", so it is in "upper Yubetsu".) (Do not confuse with Kamiyubetsu Onsen "Churipunnoyu" or Tulip Roadside Station.) Adult entry was ¥500 or about S$8.00. Unfortunately we got there late. And only had about half an hour at the place! We made the best of it, and it made our day.
Until we stopped at Saromako (Lake Saroma) visitor's centre we had no idea of where to find the flowers. So you should make that your first stop to get maps and some idea of where to go.
We plan to check out Takinoue park next.
As an aside, "Churipu" is "Tulip". The softer "T" becomes the even softer "Ch", the "L" becomes "R" and the "u" at the end is added, because Japanese don't usually like to end words with a consonant.