Question for a interior painter?

I want to paint two stripes on the top of my walls where you would put the crown moldings. I have no idea how to do it. I see videos and they kinda confuse me. i am not sure what stripe to paint first if the top or bottom one. the top one will be a lil darker than the bottom. On the videos i see people using tape to make the lines. But i read here that painters preffer or recommend not using tape. I just dont see how to get a straight line let alone two with out using tape. i guess i could make the line using that shalk string and just be super carefull painting but i dont know. any help will be appreciated. I cant afford a profesional. need to do the best i can on my own.