How to make Friends easy in Stockholm in Summer?

Answer 1

Go out to where the people are and start talking.
If the weather is good, try for a park during day and early evening.
If it is not sunny your best bet will be places that sell drinks.

If you stay in a hostel, just talk to whomever you meet there. If you stay in a hotel it will be harder, you may have to go out to meet the people.