How would I hook up this light fixture?

If you put all the wires together by color you will pop the breaker the first time you flip the switch.

So first of all if you haven't taken any splices apart, Don't! Just use the black and white that are still separate.

If you have, you will need to do the following:

Determine which set is the incoming constant hot.

Ring out which set goes to the switch.* more on them in a moment.

The remaining sets (3) go together color to color.

* The set that goes to the switch gets wired in what at first appears to be incorrect, but is actually Required by Code.

You take the White in that set, put it with the Blacks going elsewhere and the Black Constant Hot under 1 wire nut. (If they are all #14 wire a red one will do. If #12 then a large gray one.)

The black (from the switch) is now your "switch leg" and goes to the black on the light fixture.

The remaining whites all go together + the white from the light.

When you are done, you will have 4 blacks and 1 white under a wire nut.

4 whites + the white from the light under a wire nut.

And 1 black + the black from the light under a wire nut.

If you don't have a tester of any sort there is a way to find these sets without one. It's not pretty mind you, but it works.

Take any one set black/white and wire nut them together. Flip the breaker. If it trips, that's the constant hot ( C H ) set. If not try another pair.

Once you have found the C H take anyone set of wires and splice them color for color. Turn the breaker on. Flip the light switch. if it trips the breaker you have the set to the switch and can proceed with the splicing. If not, take that set out and try another set.

As I said it is not pretty but does get the job done.

Honestly, this is why homeowners with no knowledge of wiring should NOT install lighting or do any work on their electrical systems. It would take somebody with proper knowledge and testing tools to determine what the connections are in your junction box and even if the box is the right size for all that wiring (I am betting it is not). Anybody on here who tries to give you instructions for figuring this out is in the wrong. Not actually being able to trace and determine what is splice through that box and whether it was done correctly means nobody can tell you safely what should be done.

IF all 5 blacks go to the fixture then your solution should work. But, there has to be a 'switch leg' coming into the j-box from the wall switch. That single conductor and a neutral should be all that you need to connect to the fixture

Don't use all the wires till you know where they go and what they do.

And certainly don't twist 5 wires together, that's just asking for a fire.

How was the old fixture connected? That is how the new one should be installed.