First of all- well done for paying attention to your alignment! The alignment is very important in Pilates and paying attention to it will not only help you benefit from the exercises and work your body evenly (in a balanced way), but it will also develop your sense of awareness to your body and to how your body moves in space. Saying that, please never get too stressed or worried about being a little wonky… we all are… and Pilates exercises simply highlight this fact, giving us the chance to address it.
Now to your question- in the Matwork we always suggest using the space and the borders of the Mat as a reference; keep your eyes open and always look around you. Make sure you follow the instructions on where you should be on the Mat (EG Lying in the centre of the Mat, Side Lying on the back edge of the Mat, Kneeling with your hands right in the front of the Mat etc). During the exercise keep checking that you are still following the instructions given and if you realise that you have moved from the initial position then correct yourself back to it. Whenever you have your legs in front of you keep checking that they are well aligned; this is something that many people forget to do which often “throws them off”. Additionally, you can use the space (walls and objects) around you for reference; straight lines and fixation points (like when a dancer spins on one leg and uses a focus point) can help you achieve better alignment too.