lets say i have created an custom ideology and its name is "breadism" and i have an country named potato country,

i want to make potato country breadist but i cant. can you help me? btw im from another country so my english might be bad sorry

If you go to the country tab in your mod, there is an ideology editor button:

If you click that button, and then tick the green boxes for the ideologies you want adding (you cannot click ideologies which the nation already has), and tick the red boxes if you want to remove an ideology (not recommended if you still use the default ideologies)

When done, hit submit, and the new ideology should be assigned to the country.

thank you so much