Wet blocking is the most common way to block. You soak the piece in a bath of water, get the water out without wringing, then lay it out, shape it and let it air dry.
The video does it on an acrylic swatch whose stitch pattern has pulled the fabric in. The aim of blocking it is to open the pattern up. When the piece has dried under the pins it holds that shape and size, the stitches are easier to see, and the fabric is flatter and more finished.
The things to watch: keep the swishing gentle and support wet fabric from underneath, because wet stitches stretch, and press the water out rather than twisting it. Dry it with air, not heat.
1 Submerge the piece
Put the swatch or garment into a tub of water and let it go fully under. You want the water to saturate the fabric completely.
2 Swish it gently
Move the fabric around in the water for a few minutes. Keep it slow and gentle. Fast agitation can felt an animal fibre.
3 Lift it out supported from underneath
Take the piece out with your hands under it, holding the whole weight. If you lift it by one end the wet fabric stretches the stitches, and the more so with a big garment or a fibre like cotton or bamboo.
4 Squish the water out
Empty the tub, then press the piece against the side to push the water out, or squeeze it gently in your hands. Do not wring or twist it. That stretches the stitches. Press only.
5 Roll it in a towel
Open a towel, lay the piece flat on it, and roll the towel up with the fabric inside. A bigger piece takes more rolling until it is covered.
6 Press the water into the towel
Put your weight on the rolled towel. Standing on it barefoot works best. The towel on both sides takes up most of the remaining water. Unroll it and the piece is most of the way dry, which makes the drying at the end much shorter.
7 Lay it out and measure
Spread the piece on a mat, a carpet or a bed, somewhere you can pin into. Measure it against the size you want. Here the swatch is five inches tall and the aim is five and a quarter.
8 Pin it to size
Pin one corner with a T-pin, stretch the fabric gently until it reaches the measurement, and pin that corner. Match the other side the same way. For a garment, check the measurements all the way across against the finished size.
9 Pin the rest and let it dry
Add pins along the edges so nothing pulls back in as it dries. Let it dry completely. A fan speeds it up, but not a hairdryer, since heat can melt acrylic and big temperature changes can felt other fibres. Then take the pins out.