How to keep an eye on the offside and the ball going out of play

As a assistant referee

• you are on one of the offside line near any of the teams sideline
• one of the other assistant ref would be on the other sideline

There is something called a defence line also know as off side line this line is some times kept very high (closer to the halfline)

• the assistant ref should stay on the same line as the defense line and move according to the defenders moving up and down

When ball goes out for throw its the job of main ref to give the throw to the respective team the only exception is

• if the ball is very close to where the assistant ref is standing and if the main ref is far ,then its the job of assistant ref to raise the flag and give signal

how to keep an eye

• Throw-In. When the ball crosses the touch line near to the assistant referee's position, the assistant referee must make a direct signal to indicate the direction of the throw-in

• rest of the time focus on the off-side-line

check more here https://www.thefa.com/football-rules-governance/lawsandrules/laws/football-11-11/law-11---offside#:~:text=It%20is%20not%20an%20offence,and%20the%20second%2Dlast%20opponent