apparently, thistle grows great under pine trees!

Hi- I live in WI - zone 4b and I have hostas growing under my pine trees. I also have Lungwort, Sweet Woodruff, Lamium, and Heuchera. All do well as long as they get enough water.
-Sprout (Linda)

As someone who has started many beds in many places, my advice is methodical, not plant specific. You need a "big picture plan" before you start digging holes. The first step is to form a picture goal in your mind of what it WILL look like. Write some things down, start taking notes about plants you like. A spreadsheet is really cool for adding neat colums of stuff like "flower color" and "mature height", "bloom timing" whatever is important to you...

Nice-looking grass is usually a sign of at least 6 hours of sun, which is enough sun for most "full sun" plants and an ideal place for "partial sun" plants. If the label just says "shade" I would avoid those plants. Since you plan to use some larger shrubs that will make shady spots, you can later get into adding shade plants in those shade plants in the spots of shade created by the shrubs.

I would recommend starting your planting with shrubs because they are the backbone and structure and eventually they help a large garden under large trees look more properly proportioned and natural. Plant larger shrubs at the back if you will almost always look at it from a single angle, as if it's up against a fence. Or put them in the middle if you will be looking at it from all sides regularly, like an island bed. Plant according to mature height, which will seem way too far apart. It's kind of boring at first if you find that you've blown a lot of your money on shrubs and it looks like there's nothing there. But you can move perennials around and shrubs are pretty hard to relocate later.

From there, add perennials placed by mature height, so tall plants won't eventually block your view of shorter plants. You'll probably keep tweaking that every year, most people do. Add annuals to taste.

It's about impossible and a bad idea to use digging to remove a large area of grass under trees, so I just wanted to suggest that you search the term "lasagna garden" for ideas about smothering the grass, not changing the level of soil around the trees too much, the benefits of mulch, etc... If you encounter a huge tree root where you want a shrub, it's best to shift the shrub a bit than start hacking through big roots.

In addition to what sprout mentioned, I also have actea, oakleaf hydrangea, hemlocks, brunnera, Japanese Maples, joepye weed, serviceberry and arborvitae growing under mine.

tj