Montague grammar is an advanced approach to formal semantics similar to other approaches. So you might want to first start with an introductory textbook to formal semantics in general (personally I've used *Semantics* by Kate Kearns: it's useful if you can find it, though I believe it's out of print now). And then after that you can move into more advanced versions of formal notation like that. An "introductory" textbook to Montague grammar probably still assumes some general background in predicate logic, logical quantifiers, etc. On the other hand, these textbooks are typically meant to be used in a classroom with an instructor as guide. They're not easy (or necessarily intuitive) topics, in the same way that you might not do too well reading a chemistry or calculus textbook on your own. That said, these systems are designed to be consistent and logical, so in principle you should be able to figure it out in the same way you could work out calculus on your own (and in principle you could mix references to some extent, unlike say syntax textbooks), but it may be a challenge. Maybe some random youtube videos could help if you find relevant topics? There may be some translation of specific notation necessary but the concepts should be relatively standard.