Open your Google Sheet
Select the column next to where you want the new column
Right-click the selected column letter
Click Insert 1 left or Insert 1 right
To add multiple columns, select multiple adjacent columns first
Right-click the selection
Click Insert X left or Insert X right
You can also use the menu bar
Click Insert
Click Columns left or Columns right
Use keyboard shortcuts if available on your device
On mobile, tap the column letter
Tap Insert column left or Insert column right