Measure it out in paces
At home, take a tape measure and stand with your back to a wall, heels touching the wall. Take one normal step forward, then measure from the back of your heel to the wall in centimeters. This is your pace length. Let's say you measure 60cm. Now, convert the bleep test distance (20m) to centimeters by multiplying it by 100, which is 2000cm, and divide this by your pace length. If your pace length was 60cm, then 2000/60 is 33.33. That's how many paces you need to measure out for 20m. So when you go outside, to wherever you'll be training, put a bag, water bottle, or other marker down on the ground, place your toes next to the marker, and take 33 steps (or however many you calculated), then place another marker next to your toes. Your markers are now 20m apart.
Then just run a bleep test app on your phone (there are loads of these). If you're in a noisy area and have difficulty hearing the beeps on your phone, maybe use some bluetooth headphones, or put the phone half way between the two markers to make it easier to hear no matter which side you're on. (But make sure it's not in a position where you're likely to step on it while running!)
If you don't have a tape measure, an alternative is to use a ruler to measure the length of your runnings shoes. This is the same as the pace length method, but instead of using paces, you'll walk placing the heel of one foot right up against the toes of the other foot. So if you measure your shoe length as 30cm, then 2000/30 = 66.7 steps, so take 67 steps, heel touching toes, to measure out the 20 metres.