How do you scan a line of dactylic hexameter?

Take a line of dactylic hexameter in Latin, e.g.Thybris ea fluvium, quam longa est, nocte tumentem (Virg. Aen. 8.86)Begin by marking any elision in the line (where a word ending in a vowel or an ‘m’ is followed by a word beginning with a vowel or an ‘h’). In this example the ‘a’ of long(a) est. Next mark the syllables you definitely know are long, these are the syllables in which you find a vowel followed by two consonants (except in certain circumstances) or two adjacent vowels (a diphthong). I tend to work backwards through the line. In this example, the first ‘e’ of tumentem, the ‘o’ of nocte, the ‘e’ of est, the ‘o’ of longa, the ‘a’ of quam followed by longa, the final ‘u’ of fluvium followed by quam, the ‘a’ of ea followed by fluvium and the ‘y’ of Thybris are all long because they are followed by two syllables and, in the cases of tumentem and Thybris, because the first and penultimate (second-to-last) syllables of a line will always be long. So now our line looks like this:Thȳbris eā fluviūm, quām lōng(a) ēst, nōcte tumēntemThen it’s simply a job of identifying the feet and filling in the blanks. The final foot of a hexameter line will always be an anceps (a long-long or a long-short) so we have ēntĕm as the final foot. The penultimate foot is almost always a dactyl itself, which works with nōctĕ tŭm, and lōng(a) ēst is therefore a complete spondee on its own, as is (fluvi)ūm, quām before it. We are therefore left with the six syllables of Thȳbris eā fluvi(ūm) which must form two dactyls since we already have four other feet in the line, leaving us with:Thȳbrĭs ĕ | ā flŭvĭ | ūm, quām | lōng(a) ēst, | nōctĕ tŭm | ēntĕmThen we simply mark the caesura, a gap between words most commonly found in the third foot:Thȳbrĭs ĕ | ā flŭvĭ | ūm, ǁ quām | lōng(a) ēst, | nōctĕ tŭm | ēntĕm