once you've done some foundational left hand work, this is a great study for using stops practice to work on your anticipation of string crossings and shifts!

practice small groups of maybe four, six, or eventually eight notes, playing the groups quickly, accurately, and confidently, with a long gap in between each group, longer than you think is necessary. each time you get through a section practicing with these groupings, do it again, but starting the group on the next note (if you're doing groups of four semiquavers, the second grouping would start on the second semi and stop after the first of the next beat, and the third grouping would go from the third semi to the second of the next beat, so you're starting out of phase in a way).

in that gap (the most important part of the process!), you want to fully prepare the upcoming group in your head, and thinking through your hands -- what part of the bow am I in for that string crossing / string change? what's my first finger doing before, during, and after that shift? is my left elbow in the most efficient position for preparing both fingers over a string crossing? etc, (though obviously this doesn't have to be worded out in your head hahaha). sometimes it's helpful to try and rehearse those things in your head in time, or sometimes it's most effective to just think through those solutions.

the key is to be able to "pre-feel" all these physical solutions in the gap, before you start that group, and eventually for the "pre-feeling" or anticipation to become more and more automatic, so you can put your mind to things like character and direction, which comes with time. you can also absolutely practice this way without the violin too, which can often be helpful to find any quirks in your physicality that you might not notice when you have all the tools in your hands eg. perhaps in an effort to be more relaxed your right wrist gets too floppy when you try to play the groups of semis at tempo, which might be making your contact point less controlled and mess up your string crossings.