Dactylic hexameter composition

There are quite a few metric issues:

• I don't get how lines 4 and 10 are supposed to scan.
• Lines 3 and 7 seem to have one syllable too many.
• There are syllables that should be short for the line to scan: bant on line 2 and in[tra] on line 4, and -gis and -ror on line 6. The length is caused by the following word starting with a consonant for the last two.

Stylistically, you seem to end a line in a one-syllable word far more often than a Roman would. I would especially never end a line in a preposition.

Usually eum would count as two syllables, breaking your 9th line. Synizesis is possible, but you should not be relying on somewhat rare freedoms to make your lines scan.

The caesuras are fine for the lines that otherwise scan fine.

I did not pay any attention to grammar, as I took this as a purely metric exercise.

There is still a lot to learn, and the thing I would encourage you to learn next is to critically and carefully scan all your lines. If you became able to pick up most of the errors you made here, your writing will improve greatly. Making mistakes is fine; the important thing is finding and fixing them. Saepe stilum vertas!

It may also help to carefully add a macron or a breve on every vowel that is not in a diphthong. It really helps keeping track of things when you get started, and it makes reviewing your work easier for others.

I haven't checked all of it in detail – and haven't looked at things like caesurae at all; but lines 2–4, at least, are too long and/or unmetrical.

Line 2 starts out okay, but falls apart after iaciebant at the end of the 4th foot and becomes unmetrical, because the 5th foot starts with a short syllable in humo. What follows would need to be – u u – x or – – – x.

stercora lascive iaciebant humo comites de
  –  u u |–  –|–  uu|– –  | u –  u u –   –

Likewise, line 3 starts okay as a heavily spondaic line, but it's too long. Lustrato should be followed by a – x to end the line; so the 2 short syllables at the beginning of comites break the meter and make the line too long.

flaminibus missis caelo lustrato comites haec  
  – u u|–   – |–   – |–  – | – – |u u –   –

Line 4 falls apart almost immediately, even if one were to suppose that there's a hiatus instead of an elision between volcano and erant.

facti volcano erant et aspiciebant aquas e
 –  –| –  –|  u –   u  –  u u– –   u  –  –

A couple of other points:

• In all the hexameter verse I've read, I'm not sure I've ever come across instances of a line break between a preposition and its object. Perhaps it isn't unheard-of; but it strikes me as quite strange to see it occur twice in such a short passage, and not for any particularly special effect.
• As Latin hexameter developed, there was movement toward ending lines with words of 2 or 3 syllables (see, e.g., Raven's chapter about dactylic hexameter). Certainly, this 'rule' is sometimes broken; but the number of monosyllables at the end of your lines (again, in such a short passage) is somewhat odd – if your aim is a classical dactylic style.

I'll just comment on the first line. Here are two attempts to scan it. Parentheses indicate elisions.

prōveni/(am) ut sege/tis lē/g(e) amīc(e) haec / sēmina / magne.
  – ⏑ ⏑/     –   ⏑ ⏑/  –  –/     ⏑  –      –   /  – ⏑ ⏑ /  –  ⏑

prōveni/(am) ut sege/tis lĕg(e) a/mīc(e) haec / sēmina / magne.
  – ⏑ ⏑/     –   ⏑ ⏑/  –  ⏑     ⏑/  –      –   /  – ⏑ ⏑ /  –  ⏑

The first scansion, with lēge, doesn't work: the 4th foot is ⏑––, neither a dactyl nor a spondee.

The second scansion does work, so it appears that we are forced to read lege as lĕge, the imperative of legere, "to read". Is that what you intended? I'm having a hard time understanding how that could make sense in this sentence. It seems more likely that you intended segetis lēge, "by the law of the harvest", using the ablative of lex.

Even with segetis lēge, I can't understand the sentence. Provenio is an intransitive verb, but it looks like you're trying to give it the direct object haec sēmina. The best at a meaning that I'm coming up with is so far:

Great friend, I will come forth these seeds as by the law of the harvest.

which doesn't make sense in English, either, for the same reason. What meaning did you have in mind for that first sentence?