I would like to know the various obstacles that were needed to be solved in order to complete the deal.

In 1819 the U.S. had an
equal number of slave and free states (eleven each). States were generally divided along
the Mason – Dixon line. The Northern States (free) had greater population, and thus
greater representation in the House of Representatives, but the Senate was equally
divided. The population of the North was growing faster than the population in the
South, so if the two sections were to remain equally balanced, it could only occur in
the Senate, where each state was equally represented regardless of population. No move
had been made to extend slavery into territory gained by the Louisiana Purchase. That
year, Missouri wanted to join the Union as a state. Representative James Talmadge of New
York introduced a resolution that prohibited further introduction of slaves into
Missouri, and freedom at age 25 for those born into slavery after its admission as a
state. (at the time, Missouri already had 10,000 slaves.) The resolution passed the
House, but failed in the Senate, largely along sectional
lines.

Maine had applied for admission also, so compromise
was worked out:

• Slavery was excluded from
  balance of Louisiana Purchase north of 36° 30’N. (Southern Border of Missouri.) This
  appeared to be a victory for the South, as the area of the American West had been
  designated the Great American Desert, unfit for agriculture, or for anything but Indians
  and rattlesnakes, not necessarily in that order.


• Maine
  would join Union as free state, Missouri as slave state this meant that the slave state/
  free state division would remain along sectional
  lines.

Another problem arose when proslavery
interests in the Missouri Constitutional Convention insisted the State Constitution
provide that free blacks and mulattoes would be excluded from the state. (This was a
clear violation of the Federal Constitutional provision that protected "privileges and
immunities" of U.S. Citizens.) Free states were citizens of several other states. This
was an additional controversy that almost blocked Missouri from joining the
Union.

Henry Clay developed a
"Second Missouri Compromise:" The offending clause could remain but the legislature must
agree that it would never be construed so as to deny privileges that citizens held under
the U.S. constitution. (In other words, the Constitution did not mean what it said.)
Missouri was subsequently admitted to the Union as the 24th State in
1821.