Wanna know how? Follow those five steps!

1-make the the base level

2-make your other level(s)

3-make the other levels sub-levels

4-put a level link to the next level
 -if you have more than one level, just put a level link in the level that is after the base level and so on

5- publish your sub-levels FIRST, then publish the base level

So now the sub-levels should be considered as the base level. 

You got it, just that. Feel free to try!

Note: when you pause in the sub-level, it will be seen as what you did in the base level (hearted, etc..) (and also the description, reviews and the score bubbles you collected)