Step 1 - capture the SQL you are interested in. You can do this in various ways, for example, if you know the SQL_ID then

variable pls number
begin
  :pls := dbms_spm.load_plans_from_cursor_cache( 
                  attribute_name => 'SQL_ID', 
                  attribute_value => '[your SQL ID]'); 
end;

or if you know the sql text (or part thereof) you can do:

variable pls number
begin
  :pls := dbms_spm.load_plans_from_cursor_cache( 
                  attribute_name => 'SQL_TEXT', 
                  attribute_value => '%select%from%my_table%where%'); 
end;

and in either case, you can limit to the desired plan hash value.

Once you've done that, you're pretty much done - we'll use that plan until directed otherwise. New plans will be picked up, but you would need to manually evolve those plans to activate them.