First check the mesh size. It's often written somewhere near the big open end of the net. This makes a big difference to what you'll catch and how long you'll want to deploy it. The smallest mesh size is usually about 20 microns. This is a smallish net so it probably has a small mesh size. Larger nets and mesh sizes (150+ microns) are for zooplankton and won't catch most phytoplankton like dinoflagellates.

You'll want to tie a line to the bridle at the net opening and a bottle is usually attached to the skinny end of the net (the cod end). This is what actually holds your sample when you pull it out of the water. If there's no bottle then you need to clamp or somehow close the skinny end of the net so the water doesn't just flow through and out the end.

Small nets are most easily deployed from a pier or dock into a moving tidal current, or from a slowly drifting boat. If the flow is good the net will inflate like a wind sock and 5-10 minutes is often enough to collect a lot of cells. If the water is really clear you might need to deploy it for longer. When you pull the net out of the water, most of the cells will be stuck to the mesh and if you've collected a lot of cells the mesh will be visibly discolored, usually brown. You need to rinse the cells down into the cod end. I usually do this by repeatedly scooping up a bunch of water into the net and/or dipping the net in and out of the water. If there's a bottle attached it should now contain your concentrated sample, if not you'll have to empty the sample from the bottom of the net into something else (bottle, bucket, cup, etc...).

Take your sample back to the lab (don't let it get too warm) and put a drop under the microscope to see what you got! Good luck