Here are the basic steps:

Get or borrow a decent quality microphone

Decide the voicebank type you want to record: CV, VCV, CVVC or VCCV (the last is for non-japanese voicebanks). Be sure to look up the basics of each one. Some take more time to record (VCV), some are easier or harder to use in the end (CVVC and VCCV).

Look for the corresponding reclist (recordings list) for the voicebank type you've chosen. It will guide you with which sounds you need to record.

For the actual recording, I can't give you much tips as this is where I lack experience.. Several users have already shared tips though ! Be sure that your recordings are 16 bit wav files. Many people use Oremo to separate recordings.

Once you're done recording, there's the configuration process, the tricky part. Your configuration is saved in a file called "oto.ini". There are decent tutorials lying around about otoing.

Another advised step is to generate an frq (frequency) file for each wav file. UTAU does that when you play a note in UTAU for the first time. There's an frq editor in UTAU which allows you to fix bugs.

There are many things to be said for each steps, I tried to point you in the right directions. It may seem overwhelming at first, but be patient ! You will figure out everything in its own time.