Answer:
Through meritocracy and supreme success. Mussolini had humble beginnings but proved to be a very capable leader who defeated the rioting socialists and communists. Soon he began defeating the Mafia and bringing complete order to Italy. He then enacted government spending to improve orphanages, school, trains and other infrastructure and got the trains running on time. He defeated starvation and enacted strong increases in welfare to make sure the average Italian wasn't desperately poor anymore.
Soon Mussolini won wars Italy could have never done before. Then Mussolini stalls WW2 as much as possible until Hitler got too impatient. Mussolini joins the war as France and Britain turned their backs on him for hypocritical reasons. Mussolini made a smart and pragmatic decision which by supreme bad luck ended terribly. The war seemed just about over when France fell and Mussolini used that as an opportunity to redeem lands occupied by France with minimal casualties. However rhe provlem is that the US and Soviet Union went into the war which no one would have be able to predict at the time in Mussolini's shoes.
Step-by-step explanation:
Mussolini rose to power through the capitulation of the king, Victor Emmanuel. When Mussolini conducted his famous March on Rome, the military and government were well prepared to repress him and his fascist supporters. A order was brought to the desk, a order to use the military to forcefully put an end to Il Duce’s plans to rule Italy. Instead the king handed over power to Mussolini.
Now for the second part of your question, Italy joined the war to expand its borders. Mussolini was already expanding his borders, annexing Albania and invading Ethiopia.
These are the border Italy had planned on. Notice in Dark Blue Corsica and Tunisia, along with a North Western part of France. In the Balkans Mussolini planned on owning the entire coast, effectively turning the Adriatic into a Italian lake. While Mussolini and Hitler did not always get along, (Austria was allied with Italy and Mussolini did not want Germany to annex it, so Hitler sent weapons to Ethiopia to fight against him), Mussolini put that aside to try and claim more territory.