If you have you tarp with you then set it up high enough so you can comfortably sit under it. When it's raining you will have to do everything under the tarp so you want to make sure you have room (and head height) to do what you have to do.
Unless it's windy, in which case set it low so the wind doesn't blow the rain in.
Use a simple set up - just a slope in this kind of shape \ (but a bit flatter) With the back of the tarp facing the rain (unless that blocks out a lovely view, in which case it may be worth risking the odd drop of rain for a nice view - amount of wind, rain, and quality of view will dictate that). That way the rain will run off in one direction toward the back. It will be quicker to set up your tarp if you have the quy ropes already attached. You have to accept that you are going to get a bit wet when setting up if it's already raining.
Another thing to think about is where you pack your tarp in your bag. It should be on top so that you can take it out quickly in bad weather and so you don't have to take other things out first. Even better, if you have a bag with separate pouches and compartments then have it in one of those so you can take out your tarp (and associated bits) without having to open the main compartment and risk getting everything else wet.
Packing away in the rain you also have to accept things are going to get wet. Pack all your stuff whilst under the tarp and do the tarp last. Obviously. It's a good idea if you have your tarp in a wet bag rather than the non-waterproof bags they come in. That way, when you pack your wet tarp away, all the water will stay with the tarp and not leech out into your bag and soak everything else.
And don't forget a sit mat for keeping your backside dry!
Learn and practice your knots and setting up procedure before you go out and that should speed things up and mitigate any soaking.
And enjoy it! I love being out in the rain with the sound of the rain pattering against the tarp!