As a beginner, are you aware that Tunisian tends to curl? You might want to try a different stitch pattern to help counteract the curl. The only free pattern I came across is Checkered Tunisian Keyhole Scarf. The best I can tell from looking at the pattern is that the slit is worked as I thought - work half the stitches to the length desired then go back and work the other half to match. I did notice that one stitch is skipped between the halves and didn't see how it's dealt with. I think you can figure it out. Try looking at the pattern and seeing how it says to do it and then see how to apply that to your project. I hope to see photos of your progress as well as the FO!
When I was really interested in Tunsian crochet, before I learned to knit, I had a really hard time finding info and patterns online. There were books for sale but I wasn't to the point of wanting to buy books. You might see if your library has any books available.