The first issue for any astronaut answering a call of nature is their own weightlessness. They can't just sit down on the toilet and emit a sigh of relief. They'd float off again.
It all requires training back on Earth, plenty of practice, then a bit of preparation while on board the ISS. More than anything, it demands a perfect aim.
Our desperate astronaut slips his or her feet into the foot slots. It provides the anchorage to brace themselves in place. Two foam covered handles can be maneuvered across their thighs to help pin themselves onto the toilet bowl. Some astronauts simply reach up and hold onto the ceiling as well.
A clear plastic bag is inserted into the silvery receptacle to their side. It's ready to receive any tissues. Such things cannot be dropped into the bowl beneath them.
Then it's decision time. Number one or number two? They can't happen together, because each is collected and disposed of in different ways. In order to urinate, the male astronauts take a tube attached to the unit. It's placed over their anatomy and they are good to go.
A light flow of air persuades the urine droplets to continue down the tube, instead of floating away. The suction cannot be too great, as that would cause the poor gentleman an injury.
For female astronauts, the method is nearly the same, except they have further choices before they begin. Three different funnels may be attached, each shaped slightly differently in order to gain the closest match. For the ladies, the key here is to position the funnel snug against them. They haven't anything to insert into the tube, unlike the men!
Even more problematic are bowel movements. With the astronaut's rear end pressed as tightly to the toilet seat as possible, they are able to expel their waste. However, they are aiming at a hole just four inches in diameter. This takes a lot of practice to get right!