how do i reserve seats?

Trains we will be taking that take advance reservations:

one way Shin Osaka to Hiroshima (on a Friday)
one way Hiroshima to Fukuoka (on a Sunday)
r/t Shin Osaka to Himeji (not sure of day yet)

Does it make sense to reserve? I was told this was a bit benefit of the pass. Also, what do I have to do if we have a large piece of luggage (larger than carry on?)

thanks again

You can easily reserve a seat at a ticket machine of the station or an office.
If you are thinking of getting the Japan wide rail pass make sure it pays off and know that you can�ft take Nozomi nor Mizuho trains (the fastest Shinkansen) without a hefty surcharge.

For luggage if it doesn�ft fit in overhead space then reserve a seat with extra luggage area. The ticket machine will give you that option.

I bought the Setouchi pass and it does pay off. Since I bought directly from jR, I can reserve in advance but not sure how to or it its needed.

If you have bought the pass online from JR West then you should be able to reserve seats via this page https://e5489.jr-odekake.net/e5489/ibsp/CBRsvSearchSP


User guide https://www.westjr.co.jp/global/en/howto/train-reservation/

I have used it before for Kansai Hiroshima pass and it should similar work for Setouchi pass