If you’re looking for one source that will teach you Chinuk Pipa look no further. In 1924 Jean-Marie-Raphael Le Jeune, the French Catholic priest who edited the Kamloops Wawa and introduced Chinuk Pipa to BC, published the Chinook Rudiments. This is the single best source to start learning Chinuk Pipa. Period. I strongly suggest you check it out.
Make sure that you write out what you read. Focus on learning whole words in the way that Le Jeune spells them. Do not just learn the letters and start spelling words how you want. Write how you see they wrote and you will make everybody’s life easier who is trying to read your work. Spelling was very close to standardized in Le Jeune’s work and it would be a shame to lose that for Chinuk Pipa. The way we read means that we recognise the ‘shapes’ of these words, not the individual letters. If you start using different ‘shapes’ by writing with your own spelling you will be putting an unnecessary burden on people who are trying to read your work.
As a quick word of warning, a few of the ways that he writes the English-derived words in the Rudiments are not the way they were normally written in Chinuk Pipa documents. When Le Jeune adapted Chinuk Pipa to English he added a few symbols that you will never see used outside of the Rudiments and when he writes English in Chinuk Pipa. Indigenous authors, to the best of my knowledge, did not use the English-adapted system when writing English-derived Chinook words and neither did Le Jeune (so neither should you).
Once you have some of the words down, I recommend jumping into some letters that have already been transcribed so that you can check if you get stuck. Check out, for example, Catherine Isaac’s letter, the Alkali Lake letters, and if you want something substantial check out the 64 page long report on Father Le Jeune, Chief Louis, and Chief Chilliheetza’s trip to Europe (the accompanying Kamloops Wawa can be found here).
After that you can just hop in and start reading whatever you want! Here’s some links: