Get some solder paste. Look on youtube for soldering videos.
If you've ordered from JLCPCB, you may get a solder paste stencil with the boards. If so, just use that to spread solder paste. To keep the stencil flat, it's best to support the whole of the stencil at the same height, so you need something around the board that is the same thickness as the board. I use some old or unused circuit boards around the board being pasted, but anything 1.6 mm thick will do. Use an old credit card as a squeegee.
If you don't have a stencil, make sure that the solder paste is in a syringe, which is how small quantities arrive anyhow. They often come with a suitable blunt needle for dispensing. Put a small blob of solder paste, really small, maybe 1/2 mm diameter on each pad.
Use tweezers to place the components into place. The solder paste will keep them in place if you handle them carefully.
Now you need to heat the board so that the solder melts. One method is a hot air gun. The type used for paint stripping is fine. You want about 300 °C, but if it's hotter than that, keep it high above the board and move it slowly closer. Keep the airflow low to stop blowing the components off.
An alternative is an old frying pan, and place the boards in that and then on a stove and slowly bring the temperature up.
With either heating method, make sure you have light and whatever magnification you need to be able to clearly see the components as they are heated. The solder paste will initially become more runny, and then dry out, and then melt. As it melts, it should flow onto the pads of the board and the connections of the components. The components will be pulled into place by the surface tension of the molten solder. Remove the heat source and leave until cold.