Be the pocket. Practice grooves and lines against a steady beat and see whether you're rushing or dragging behind it.
Steady time is the skill every other skill sits on. Practising bass with a metronome trains your internal clock, exposes the spots where you rush or drag, and makes everything you play feel tighter. Drumolo's metronome is free, runs in your browser, and shows your timing visually so you can actually see your progress.
Yes — Drumolo is a free online metronome that works perfectly for bass. Open it in your browser, set the tempo and start.
Start slow enough to play it cleanly, then raise the tempo a few BPM at a time. Clean and slow beats fast and sloppy every time.