Step 1: Turn on motion controls if you haven't already.
Step 1.5: Stop dying.
Step 2: Tell us what weapon you're using.
Step 2.5: Whenever you're on Tower, just use a Luna Blaster variant. It's a pretty easy weapon to use, especially in B+ rank towers. That's only if you want to take the easy route out: alternatively, you can just get better at the game.
Step 3: Start flanking. Start doing it better. Stop getting flanked. Do it better. (to be less facetious and more specific, know every map's power points, especially ones that compliment your weapon or the objective)
Step 4: Always have an escape route ready in case you get surprised.
Step 4.5: Stop dying.
Step 5: Unless your weapon is a special-oriented weapon, get to the objective as fast as possible. Don't die though.
Step 5.5: Stop dying.
Step 6: Don't stay in one place, even in splat zones. aka take the initiative, being too passive will just let the other team get the objective. But if you're whole team gets killed, don't go YOLO. Back off.
Step 6.5: Stop dying.
Step 7: Maybe you just suck with the weapon on certain maps/modes. Either train it more through grinding ranked/turf or just choose another entirely. ALSO: in B+, there's no such thing as a bad weapon (unless you use a Nozzlenose or maybe an inkbrush, but even then). There are weapons that are easier to use than others, but it's possible to find success with any weapon.
Step 7.5: If you died, think about WHY you died and how you can avoid it later.
Step 8: Stop holding grudges. Stop blaming lag. Stop blaming you're teammates. B+ is hard, but it's still possible to compensate for bad teammates. Be consistent enough where you don't have to rely on it. Be consistent enough where lag becomes less of a wild card.
Step 9: Know you're goddamn weapon (# of shots it takes to kill, how much ink your subs take up, the exact range of you're weapon, etc). Know the role of everyone else's weapon (see previous bracket). Kill weapons you can counter. Avoid like the plague any weapons that counter you. Or take the risk and surprise them.
Step 10: Get better gear. Or better yet, get gear that fits with your weapon more. Not super necessary, but it makes life easier.
I can give you more specific advice if you give your weapon and mode/map you're struggling in.