One good source of info for water sampling is the USGS field manual:

https://www.usgs.gov/mission-areas/water-resources/science/national-field-manual-collection-water-quality-data-nfm?qt-science_center_objects=0#qt-science_center_objects

Chapter 2 has some info on filtration (see attached files)

Make sure before you purchase too many sample containers etc, that the supplies are appropriately metal-free. This applies to filters as well, and the acids you will need to stabilize the trace elements in the sampled water. Check the lids too: liners may be treated with something that might look like field contamination, but was in the flexible disc inside the screw-on lid.

As you say you are new to hydrochemistry , I would recommend you speak to a local expert (university researchers or government environment office etc), see what equipment they use etc and get an idea of what you need for your purposes.

Also speak to the analytical laboratory that will be doing your analysis and find out what they need in a sample: they may have done this work before and will have some suggestions that could save you a lot of time, travel, and plastic containers if you haven't thought of some important details.

For good filtration it is better to use 0.45 millimicron filter paper.

0.45 µm Millipore filter paper is very good for filtration water samples in the field.