If you wanted both parts of the verse in the same diagram, you could join them with a dotted line, something like this shortened, English version: This makes clear that each phrase could be its own sentence, as in some translations. The direct object is added after a vertical line, just as a direct object would be after a main verb: And there it is. Here the action of confessing gets its own direct object (‘their sins’). Tradition uses a curved line for a modifying participle, but I’ve substituted an angled one, with the participle extending across the angle. What does it modify? Who is confessing their sins? ‘They’ are, so the phrase goes under the subject. We need to examine this phrase a bit further. The final piece (other than the introductory καί, which I will ignore) is a participle phrase: ἐξομολογούμενοι τάς ἁμαρτίας αὐτῶν. Since our purpose is clarification of the underlying sentence structure, I feel that this is unnecessary clutter. In an all-out version, both τῷ and Ἰορδάνῃ would be further separated out as modifiers, and placed on slanted lines below ποταμῷ. A second prepositional phrase now follows, describing the location of the baptisms: Note that these are simplified diagrams. At any rate, the ὑπό-phrase modifies the verb, and is placed as such: I am following the traditional practice for prepositional phrases, where the preposition is placed on the slanted line, and the object of the preposition on a horizontal line. From the previous verse we can identify this individual – doing all the baptizing – as John the Baptist. With a passive verb, the grammatical subject is the recipient of the action performed by the active doer. It does not need to be expressed separately in Greek, and I have added it here in English, to make it clear that you won’t find this word in the original verse: With a passive verb you often find a ‘by’ phrase that is, a prepositional phrase using ὑπό and naming the active doer. The subject is ‘they’, i.e., the people from the country of Judea, and all Jerusalem. ‘Eβαπτίζοντο can be a sentence on its own in Greek, because it includes the subject within itself.
This is the 3-P, imperfect middle/passive indicative of βαπτίζω, ‘I baptize’ thus, ‘they were being baptized’. The second part, in blue, will be diagrammed below.
Here’s the entire verse, with an English translation.