The entire verse, again:
οἳ καὶ εἶπαν Ἄνδρες Γαλιλαῖοι, τί ἑστήκατε βλέποντες εἰς τὸν οὐρανόν; οὗτος ὁ Ἰησοῦς ὁ ἀναλημφθεὶς ἀφ’ ὑμῶν εἰς τὸν οὐρανὸν οὕτως ἐλεύσεται ὃν τρόπον ἐθεάσασθε αὐτὸν πορευόμενον εἰς τὸν οὐρανόν.
The final part of the verse reads:
οὕτως ἐλεύσεται ὃν τρόπον ἐθεάσασθε αὐτὸν πορευόμενον εἰς τὸν οὐρανόν
There are three verbs:
The first is ἐλεύσεται, the 3-S, future middle indicative of ἔρχομαι, 'I come, I go'. Remember that this irregular verb is deponent in the present and future systems, although not in the aorist.
The subject of ἐλεύσεται is understood to be Jesus from context. The verb is introduced by οὕτως, an adverb meaning something like 'thus', 'in this way'.
So Jesus 'will thus come'. In theory, ἔρχομαι can also mean 'I go', but is more commonly seen as 'I come'.
The second is ἐθεάσασθε: 2-P, aorist middle indicative, θεάομαι - 'I see, look upon'
The second person plural indicates that the two men are still addressing 'you' - the apostles. The apostles were looking at something.
Αὐτόν is the direct object of this verb - 'you were looking at him', i.e., Jesus.
The two words ὃν τρόπον correspond to something like 'that manner, which manner'. English translations almost always add 'the', and typically translate the phrase as 'in the same manner', although the King James avoids 'the' by reading 'in like manner'.
What manner are we talking about? This requires that we look both forward and backward in the verse. But before that makes sense, we need to look at the final verb and translate the remainder of the verse:
αὐτὸν πορευόμενον εἰς τὸν οὐρανόν
The third verb form is πορευόμενον, a masculine singular accusative, present middle/passive (deponent) participle of πορεύομαι, 'I go, I journey'. It refers back to αὐτόν, and thus to Jesus, who went εἰς τὸν οὐρανόν, 'into the sky'.
With ὃν τρόπον the writer is apparently talking about the manner that Jesus was taken up into heaven (ὁ Ἰησοῦς ὁ ἀναλημφθεὶς ἀφ’ ὑμῶν εἰς τὸν οὐρανὸν), which the apostles saw.
By this time, although the English ends up being fairly straightforward, the Greek may have gotten confusing. To recap:
Jesus was taken up into the sky, from you.
Jesus will come (back).
Jesus will come back in the manner that you saw him.
Jesus will come back in the manner that you saw him going into the sky.
And a rather rough final translation:
and they said, 'Galilean men, why do you stand looking into the sky? This Jesus who was taken up from you into the sky thus will come / return in (the) manner you saw him going into the sky'