Ttdfjt

Essentially, I want to say something like:
"If you read this, your will will be mine". (In a teasing way like, Who Reads This Is Stupid).

I 'distilled' it as much as possible to "reader beware: your will is mine". Which I (probably falsely) remember to initiate with "Cavea t, Lector...".

But forums and Google Translate show me different words, leading me to compile nonsense like:

'cave lectorem tuus anima mea'

Partial translations I find online refer me to "your soul will be sorrowful".

If anyone may please assist, with any verbs, tips, help or pointing me to the correct dictionaries/grammar: I will greatly appreciate every bit of input.

'cave lectorem tuus anima mea'

This is pretty close! Just some case and agreement issues.

Cave means "beware!" as a command to someone; caveat means "may [he/she] beware". So if you use cave you're talking to the person directly (cave canem "watch out for the dog!"), and if you use caveat you're talking about them (caveat emptor "the buyer should be wary").

If you want someone to avoid an action, the usual phrasing is cave ne plus a verb in the subjunctive. (Formally, this is called a "clause of fearing".) If you want them to fear a thing, the thing goes in the accusative. Here, it seems like you want neither: just a general "you should be afraid".

Lector ("reader") should be in the vocative if you're addressing them directly, or in the nominative if you're talking about them. Conveniently, the two forms look exactly the same for this word!

"Your soul" would be anima tua; "their soul" (if you decide to use caveat) would be anima ejus. The second word can be left off if it's clear from context.

"Will be mine" is literally mea erit; you might also use ad me veniet, "will come to me" = "will be mine". (I most likely got this phrasing from the Requiem mass: ad te omnis caro veniet = "to thee all flesh shall come".) If you want the subjunctive, that's veniat, with an a, or sit for erit.

All in all, my recommendation would be caveat lector ne anima [ejus] mea sit. "The reader should be wary, lest their soul be mine."

The closest I can come is

Cave lector, anima tua mea erit.

means "Beware reader, your mind (or spirit, or soul) will be mine". I don't quite see how that adds up to "Who reads this is stupid", but maybe you can.

You might even consider something like Hoc lecto tua voluntas mea fiet, using an ablative absolute. Note I'm using voluntas rather than anima, too.

Perhaps even just Hoc lecto meus fies, to be more compact and not as Englishy.

If you'd like to say "If you read this, you will be mine" as literally as possible, this is probably as close as you will get:

Si hoc legis, mihi futurus eris.

Esse+Dat. means to belong to somebody by the means that you are somebody's property. Moreover, using a conditional clause shows that if you don't read it, you won't belong to the writer, but that as you've now read it, you don't stand a chance and there is no point at being careful about it, which is why I prefer this solution over something with cavere. As pointed out by @MPW, using the ablative absolute is also an alternative you could think of - especially if you want to stress that now that the reader has read the text, he is already your property.