REW has a good help section. REW has many functions and options, so it can take some effort to read all the relevant descriptions, and defer reading about features that you initially won't be interested in trying. You would typically use the Generator and RTA functions for manual observation, and use the Measure function for automated sweeps. The Preferences function is the starting point for selecting the soundcard and driver (hopefully there is an ASIO driver available) and the soundcard channels you want to use as output and input. The
https://www.avnirvana.com/ website has an REW forum, and is also where the software is available for download. Some forums have a strong use of REW and threads such as
https://www.diyaudio.com/community/forums/software-tools.123/. Googling well known soundcards (like EMU0404, and Focusrite 2i2) brings up posts/threads where the soundcard has been tested using REW (and others) and indicates how they have been used and setup. Googling tutorial may also be worthwhile - I don't look at the youtube posts but I can imagine there are a few helpful ones.
The 2019 review link (
https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-asus-xonar-u7-mkii-adc-dac-hp.8165/) indicates your card does have an ASIO driver available, but it sounds like there are issues, and the ASUS site only shows software up to 2016. Hopefully you have a Mk2 as that has later software. If you can get ASIO with operation at 192kHz at 24bit then that is great start. Some soundcards are just fair performers, in which case you may need to spend time to characterise their performance for DAC and ADC paths to appreciate how to get a suitable performance for your application and to appreciate any limitations (eg.
https://www.avnirvana.com/threads/measuring-frequency-response-and-distortion-of-amplifiers-%EF%BC%884x150w-rms-and-4x80w-rms%EF%BC%89.9764/).
If you are lucky enough to have a soundcard and ASIO driver that operates 'norminally' with REW, then that can open up a lot of measurement functions that imho far exceed legacy test equipment like scopes and distortion analysers and signal generators and impedance analysers/LCR meters, within the bandwidth constraints of typically 2Hz to 96kHz.
As an aside, I recently did a quick impedance check on a 1-to-9999 ohm 1930's 0.1% tolerance resistance decade box that I just restored, and due to the bifilar wind of the manganin coils and layering it seems like phase shift is fairly low out to 100kHz (circa 10deg at 30kHz). So I hope to do some more investigation on how practical it may be to normalise for effectively zero phase shift up to 100kHz - all with the help of my soundcard and REW.