Lay the rim on your kitchen table and loosen and remove all the nipples without removing any spokes from the rim. Your front hub is already broken in to the spokes and you don't want to change that unless you're installing new spokes. Take note of how many spokes each individual spoke crosses between the hub and the rim. Once you have all the nipples removed and the rim is kinda laying flat on the table, rotate the hub slowly (clockwise or counterclockwise, doesn't matter) and the spokes will pull out of the rim and start to close up like a salad spinner. Just turn the hub enough to let the spokes barely pull out of the old rim. Keep in mind that as you begin to turn the hub, the spokes pointing in the direction you are turning are going to get long first, and the ones angled the other way will come away from the rim instantly. Once they're all clear, take the new rim and lay it on the table beside your original and set it in the same orientation so the angles on the spoke holes are pointing the same way as your original. Then carefully pick up the old rim and toss it on the floor being careful not to disturb the spokes. Pick the new rim up, maintaining the correct orientation, and place it exactly where the old one was. If you didn't move the spokes too much they should all be pointing at their proper new homes. Don't try to rotate the hub to expand them again or you'll make a mess. Take each spoke, one by one, and pop a nipple through the rim and just spin it on a thread or two so it doesn't fall off. Once you have all the nipples on you can turn the hub to spread things out and start tightening things up. A good general place to start when truing up a freshly laced wheel
is to thread all nipples on the same amount, then gradually tighten in steps,1/2 turn for every spoke, until things aren't too rattly. Then you can pop the wheel back on your bike and spin it and start to true it up. If it's really wobbly side to side you'll need to straighten that out a bit, but you want to get the out of round or up and down out first. Spin the wheel, hold a marker against your fork to steady it and gradually bring the tip closer to the wheel until it starts to touch the high spot in the rim. Always loosen the spokes on the low side a bit first and then tighten the high spot spokes by about the same amount. You adjust the spokes in the middle of the bump the most and then working out from those, adjust their neighbors gradually less. Remember that there is usually a little bump in most rims where there are welded and you'll never get that out. Once you get the out of round nice and straight you can work on the side to side the same way, loosening the spokes on the pulling side first and then tightening the spokes on the opposite side. Once everything looks straight check for loose spokes by tapping them with your spoke wrench. They shouldn't have a dead sound and if you did a good job they should all have about the same tone. Good luck.