Short answers, pulled from the story.
The Missouri General Assembly passed the Military Bill on the 11th of May 1861. Governor Claiborne Fox Jackson used this act to disband the old Missouri Volunteer Militia and reform it as the Missouri State Guard.
Sterling Price received a commission as the first Major General for the Missouri State Guard on the 15th of May 1861. He served as field commander while Nathaniel Lyon took command of the opposing department later that month.
Price defeated a smaller Union force under Nathaniel Lyon at Wilson's Creek on the 10th of August 1861. The battle killed Lyon and drove back his army, allowing Price to continue operations against Federal troops.
Authorities estimated that at least 34,000 Missourians served in the Guard by 2007. Probably close to 40,000 men served at one point or another with strength peaking between September 1861 with about 23,000 to 28,000 men.
On the 17th of March 1862, Price merged the Missouri State Guard into the Confederate Army of the West. Former troops formed the core of the Army of Missouri which participated in Price's Missouri Expedition in 1864.