Do you understand correspondences well? Like, could you think of appropriate candle colors and incense and number of candles to use? If not, you should stick to pre-written spells. But if you understand correspondence, you understand the bulk of spellwriting.
Making your own spells is usually helpful because you pour into them your personal energy therefore they usually work better for you. But surely you need to start with the basics and some energy manipulation before making a spell. But here is a way to make your own spell.
You will need
* fine paper
You write your spell usually in a rhyme. Let the words flow freely from your heart something you feel emotionally connected to. Something that you know it is yours and yours alone. Using a blessed pen is working for some people but I don't believe you really have to. After finished you visualise the letters glowing in a white light and you call upon your God, Goddess or the elements or whatever you may believe to bless it. Use it wisely with respect.
I don't understand why you would want to bless a pen, but writing a spell is easy. It is pretty much just stating your intent and then build up from that. Add some words, some ingredients, etc that matches your intent, like Falling explained more poetically.
After you write your spell, you must know how to raise and send energy to beget your intent. If you don't know how to do this, start with meditation. Become aware of the energy around you and in you. Make a connection to the world the surrounds you.