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I'm not entirely sure where to start.

I'm kinda thinking it could be maybe a bit more simplistic than Spanish, but... I'unno. All you guys who have made sister languages to NatLangs, how'd you do it?

Also, I want it to be definitely recognized as a relative of Spanish.

D'you think I should put this on 'til I become better at Spanish, or maybe take a Latin course?

I've got a few tips:

1) Steal a lot of Spanish vocabulary. You don't have to keep the exact words, but developing a simple but consistent method for turning Spanish words into words for you conlang will help you have a reliable lexicon that both works AND is reminiscent of Spanish. This is probably the easiest way to make your conlang similar to Spanish.

2) Don't try to copy the grammar! NatLangs are often much, much more complex in grammar, especially in less obvious grammatical fields that fall in the grey area between "solid grammar" and "idiomatic structures." You're better off just keeping the typical Spanish word order, but like Homurakitsune said, simplify your grammar.

3) Add one or two new sounds and remove one or two sounds. But don't change them arbitrarily... You want the changes to be systematic, such as "When this sound occurs in this way in Spanish, in the conlang version it would be this other sound" or something along those lines. Just like with the grammar, you want the changes to be somewhat traceable, as if a significant change in culture caused the language-wide mutation out of Spanish.

4) Try to avoid influences from completely unrelated languages. For example, try not to add in Chinese concepts, or Russian words, or anything like that. If you want it to still be obviously similar to Spanish, stick with influences from primarily Spanish (duh) and then, if necessary, pick lightly from closely related languages like Portuguese.

5) Segue from #4! Look at the differences between closely related languages like Spanish and Portuguese. Try to spot where certain sounds are systematically and reliably different. You should be able to take a Spanish word, and say "You know, I bet if I change this sound here, it will be the Portuguese version of the word!" and that's the kind of change you want in your conlang.

6) Don't forget to add in some quirks so that it still kind of seems like a natlang. A few unexpected irregularities are never a big problem and can really add to a conlang's "personality." Just try not to add things that make absolutely no sense and would make the conlang more difficult to speak/read/write/etc.

7) Use the same verb system, but keep it a LOT more simplified. I've spend 4 consecutive years of Spanish studying in school, and by year 4 you realize there is almost no end to the flood of complicated verb forms and usages. Don't make it so that a speaker has to have memorized entire charts just to use a verb in different ways. Also, Spanish has 3 basic types of verbs ( -ar, -er, -ir ), which automatically complicates things. Reduce it to two, or even one, or at least give the endings some meaning beyond simply determining which chart of tense endings to use. An example of that would be to make -ar verbs indicate transitive, -er to indicate intransitive, and -ir to indicate verbs that work both ways (just one possibility, though).