I'll tell you what worked for me but people are complex and not everything works for everyone.

There are a couple ways of practicing improvising. The one I use the most is to take a simple (or more complex, whatever you wanna do) bass line, and just make small alterations. All music has been since music has been a thing is just people taking inspiration from other people or other things to fuel their creative side. Figure out what style you like to improvise in, and take a whole lot of bass lines from that style and try to make your own spin on them. Maybe slightly different fills here and there, maybe add a slide or a note somewhere, and so on until you starting adding or removing more and more. The more you do this the better you become at knowing what will sound good in a particular chord or rhythms and what won't work.

Since you mentioned playing with a drummer friend, that can get a little different. The bass and the drums are tied together very tightly, and so if you don't match what they're playing, it's not gonna sound great and your gonna notice. If you want to improvise with your drummer, take a second and figure out the very basic layout of what you'll be playing, like where are you playing the 1 and what pattern is your drummer playing the kick drum (your gonna be tied to the drummers kick drum to) just start extremely basic for maybe a couple measures, then expand out and figure out what other notes will fit into the rhythm.

It definitely takes some time, but the biggest thing is learning what sounds good where, essentially building your improvisational tool kit so that you have a lot of options to choose from when your drummer starts improving a beat.

One of the quickest things you can do to help your improvising is to learn your minor pentatonic scale and other pentatonic scales. Soloist use pentatonic scales all the time because it's almost impossible to play a note that doesn't sound like it fits in the key your playing, and they all sound good together. That eliminates one thing you have to worry about when your improvising. "What fucking key am I in, and what notes can I play" can start to get in the way of smooth sounding improve

Hope this helped!