There're no real strict guidelines on build, so the best we can really do is give you tips.
First of all you want to know what you're going to be asked to do in your team's scheme, or you want to build toward a certain type of scheme and lobby for it. Especially in the case of safeties this is important, because they're asked to do so much. More often than not most people will play some sort of Cover 0/Cover 1 scheme, which results in the SS playing a hefty dose of man coverage as well as be asked to roam and do backside pursuit, or in some schemes even blitz.
This basically means you'll want a certain level of speed to match up with opposing RBs, FBs and TEs, because those are your primary concerns. Especially TEs. And you'll want a certain level of agility to bring out the best of your speed, and to let you change directions quickly (in the case of man coverage as well as pursuit)
I know a lot of successful SSs with ridiculously high speed, and I've seen a lot of SSs with really, really low speed, but you probably want to have a close number of speed and agility. SS tend not to be used as the deep safety cover, so you can USUALLY rely on your FS to try to cut down homerun plays. Again, this depends on your scheme though, as I know quite a few schemes that actually played more of a cover 2 (not precisely though) setup, with both safeties chopping down big plays.
Strength, agility, vision, and confidence affect your ability to make the tackle, and the tackling skill affects your ability to tackle and the quality of the tackle you make. Essentially, you can go like the 40-40 way (just random numbers) and have an even, low split to try to miss as few tackles as possible, I've also seen some people with high speed, low strength (30-35ish) and high tackle (50-60) in order to try to force fumbles, and I've also seen some people with like 50 strength and 35 tackle who hoped simply to guarantee a tackle on every sort of WR/RB/TE/FB.
You'll also want a high level of vision to read plays (so you get where you're supposed to) and you'll want jumping to knock down passes and/or get picks.
Most people softcap, at least the first softcap, speed, agility, and vision, as a bare minimum, and I'd really suggest the same. Otherwise the rest is up to you and what is expected of you on your specific team.