It works exactly like it does in the US. By apply, somemom means contact the PI, see if he/she has openings and ask if you can volunteer in their lab.
(Note I said volunteer. Don't expect to be paid, esp if you're not currently attending the institution the PI is associated with or are not part of a sponsored program like REU. Profs are always short of $$ and like to save their funding for more important things than paying a summer student from another school.)