Personally I would go back and do the tutorial and then just experiment until you find that you can build easy to fly planes reliably. But not all plane configurations are easy to fly, so take that into mind

Because you skipped the tutorial, that's why.

Ive tried two of your models (the red one with three engines and the green one) just for fun, and neither have ailerons or any other method of roll control. They are stable enough, but without a method of controling roll they are, well, difficult to fly.

because you need to have horizontal stabilizers too, and take into consideration the weight and lift centers

If I control my planes are hard