Hopkins was an advocate of sprung rhythm, and many of his poems are written in it. (Others are written in conventional poetic meters.) In Hopkins' sprung rhythm, every line has the same number of stressed syllables, but the feet are irregular. Hopkins put accent marks on syllables when he didn't think the stresses were obvious. For example, in Pied Beauty, Hopkins put accents on only two words, áll trádes, where he wants a one-syllable foot:
Glory be to God for dappled things—
For skies of couple-colour as a brindled cow;
For rose-moles all in stipple upon trout that swim;
Fresh-firecoal chestnut-falls; finches' wings;
Landscape plotted and pieced—fold, fallow, and plough;
And áll trádes, their gear and tackle and trim.
All things counter, original, spare, strange;
Whatever is fickle, freckled (who knows how?)
With swift, slow; sweet, sour; adazzle, dim;
He fathers-forth whose beauty is past change:
Hopkins said the rhythm was paeonic, by which he means the feet on average have four syllables. It's clear from this that every line, except the last, should have three stressed syllables. For most lines, it's fairly clear how to do this. Here's how I would scan it:
Glóry be to Gód for dáppled things—
For skíes of couple-cólour as a bríndled cow;
For róse-moles all in stípple upon tróut that swim;
Fresh-fírecoal chéstnut-falls; fínches' wings;
Lándscape plótted and pieced—fold, fállow, and plough;
And áll trádes, their gear and táckle and trim.
Áll things counter, oríginal, spáre, strange;
Whatéver is fíckle, freckled (whó knows how?)
With swíft, slow; swéet, sour; adázzle, dim;
He fáthers-forth whose béauty is pást change:
I'm fairly sure I've scanned most of the lines correctly, but there are a few lines where I don't know whether I've done what Hopkins would have wanted.
For example, I'm fairly sure that the first syllables of Landscape and plotted should be stressed, but I'm not at all sure whether to put the next stress on fold or fallow. And I don't know whether Hopkins would have cared which of these was stressed, or not. After all, he didn't mark this line for the reader.
And there are lots of people who read this poem without following Hopkins' rules of sprung rhythm and make it sound great. Just don't read it as if it were written in iambic pentameter/hexameter. It doesn't sound as good, and we're sure that wasn't Hopkins' intention.