Slow motion practice, with a metronome.
Practice making the chords in very, very slow motion, such that you can make it the way you want (cleanly, each finger down at the same time). You will be going very, very slowly at first. For example, it might take you 10 seconds to make the chord cleanly, very slowly moving the fingers, paying attention that each finger is doing the right thing. Don't worry about the speed now. It's more important for your movement to be right than it is to be fast. Fast is going to come with practice.
Alternate between not making the chord, with your fingers off of the instrument and your hand relaxed, and making the chord. Very slowly, very perfectly. Each time you've made the chord, pluck the strings to make sure you've got a clean sound. This checks for finger placement and pressure. When you're not making the chord, remove your fingers from the shape they were in.
Now that you can make the chord cleanly, we're going to work on speed. Put on the metronome and make the chord, and remove the chord, to the ticking of the metronome.
When you can do it effortlessly at a given speed, then speed up a bit. When a speed gives you trouble, slow it down.
You'll repeat this in each practice session. Don't stay on one chord for too long during each practice session--there are diminishing returns. It's better to do this exercise for, say, 10 minutes over four practice sessions in a day, than it is to do it for 40 minutes in one practice session.
This exercise also work for changing from one chord to another. For example, the key of C is very common in ukulele, and in the key of C, you'll use the F and G chords also. So you might practice changing from C to F to G to C (or other sequences of these chords) using the same technique. Go very, very slowly, get it right, then slowly speed up.