Both your sentence and your question could do with improvement.
If you are comparing two options, you should ask which is more suitable, not most suitable.
Although both follow and stick to are possible in this context, one generally follows a plan and sticks to a regime/schedule/one's guns. That's to say, stick to implies that elements such as courage, determination, will power and persistence are involved. Your choice would depend on the context.
Make a plan for your day is correct English, but simply plan your day is idiomatic.
In your sentence, follow / stick to it sounds more as though you are talking about your day than your plan.
I would suggest:
You should plan your day and follow that plan for at least a month.
You should plan your training schedule and stick to it for at least a month.