Please explain how the Israelites were supposed to keep the Sabbath year Leviticus chapter 25 describes a "Sabbath year" for the land. Every seventh year the Israelites were not supposed to prune their vines nor plow or plant their fields.


  Lev 25:3-7 (NET) (Lev 25:3) Six years you may sow your field, and six
  years you may prune your vineyard and gather the produce, (Lev 25:4)
  but in the seventh year the land must have a Sabbath of complete rest
  – a Sabbath to the LORD. You must not sow your field or prune your
  vineyard. (Lev 25:5) You must not gather in the aftergrowth of your
  harvest and you must not pick the grapes of your unpruned vines; the
  land must have a year of complete rest. (Lev 25:6) You may have the
  Sabbath produce of the land to eat – you, your male servant, your
  female servant, your hired worker, the resident foreigner who stays
  with you, (Lev 25:7) your cattle, and the wild animals that are in
  your land – all its produce will be for you to eat.


Apparently the Jubilee year is every seventh Sabbath year, but has additional features. If I have misunderstood the text (I think the 49th year counts consecutively and the 50th counts inclusively so they come out the same) please let me know that too. The text does say they could eat the "Sabbath produce", I assume that was whatever grew by itself from the prior year's planting, yet it specifically excludes any grapes.

God does say, in verses 19 to 22, that He will increase the harvest of the sixth year. I wonder how practical this is, and to what extent a miraculous harvest would be needed to provide enough to sow the land and eat through the eighth year. Is there anything in the Bible or history or tradition that would explain this better?
Following the commandments of God requires faith and commitment. While God forbids them to plough the land on Sabbath Year(shemiṭṭah), God also promised them to bless abundantly on the sixth year.


  You may ask, “What will we eat in the seventh year if we do not plant
  or harvest our crops?” I will send you such a blessing in the sixth
  year that the land will yield enough for three years.
  (Leviticus 25:20-21, NIV)


This is the same case for Tithe. While giving 10% of our income to God may seem to be a lost, we are actually gaining. All we have to do is test Him in this. 


  Bring the whole tithe into the storehouse, that there may be food in
  my house. Test me in this,” says the Lord Almighty, “and see if I will
  not throw open the floodgates of heaven and pour out so much blessing
  that there will not be room enough to store it. I will prevent pests
  from devouring your crops, and the vines in your fields will not drop
  their fruit before it is ripe,” says the Lord Almighty. (Malachi
  3:10-11, NIV)


It was also not easy for the Jews who settled in Palestine during the Zionist movement to observe the Sabbath Year but they somehow managed to observe it.


  Since the Zionist movement began to encourage agriculture in
  Palestine, the observance of shemiṭṭah has become a problem for
  solution. The leaders of the movement, who had the interest of the
  colonists at heart and feared that the shemiṭṭah might jeopardize
  their existence, claimed that the law is now obsolete. (source)


There are also some practical stories where those who observed the Sabbath Year were blessed miraculously.


  Haredi Jews tell stories of groups of Israeli Jews who kept the
  Shmitta and experienced remarkable agricultural events which they
  describe as representative of miracles in fulfillment of the Biblical
  promise of bounty. One famous story is told about the
  then-two-year-old village of Komemiyut during the 1952 Shmittah. The
  village was one of the few who refrained from working the land that
  year. At the end of the Shmittah, farmers searching for seed to plant
  found only wormy, inferior seed that had been rotting for years in an
  abandoned shed. Rabbi Binyamin Mendelson advised them to sow this seed
  anyway, saying "The Almighty who causes wheat to sprout from good seed
  will bless your inferior seed as well," even though it was three
  months after neighboring villages had planted their fields. They did.
  That year the fall rains came late, the day after the Komemiyut seed
  was sown. As a result, the neighboring villages had a meager harvest,
  while the village of Komemiyut, who sowed from the old store, had a
  bumper crop. (source)


As for the 49 or 50 years cycle, Jewish Encylopedia says


  There is a difference of opinion in the Talmud as to whether the
  jubilee year was included in or excluded from the forty-nine years of
  the seven cycles. The majority of rabbis hold that the jubilee year
  was an intercalation, and followed the seventh Sabbatical year, making
  two fallow years in succession. After both had passed, the next cycle
  began. They adduce this theory from the plain words of the Law to
  "hallow the fiftieth year," and also from the assurance of God's
  promise of a yield in the sixth year sufficient for maintenance during
  the following three years, "until the ninth year, until her fruits
  come in" (Lev. xxv. 22), which, they say, refers to the jubilee year.