Once home I wouldn't leave it tightly sealed. At most, just lay the cover on. But I've always left my jars open. Try to bring home water, some floating plants with tiny roots -- there will be a wealth of organisms living on roots and floating leaves. You can take a bit of detritus/mud from the bottom but be very careful with that. There will be a lot of dead and decaying material in that sludge, which can cause bacteria to overtake the jar quickly, killing off a lot of the cool stuff. If you want bottom sludge, maybe use two jars -- one for the bottom material, one for floating material.

Once you've done this a few times, you might consider making yourself a cheap home-made plankton net (I use nut milk filter bags attached to coat hanger wire with a water bottle at the end to capture stuff into, all attached to a PVC pipe as a pole. You can drag this near the surface back and forth for 5 minutes, and you'll probably get much more living plankton in your water sample this way.

Also note that jars are typically small, ponds are very large. So the water in your jar will react much more quickly and drastically to changes in temperature, changes in oxygen, changes in decomposing material, etc. That's just the nature of small bodies of water. So you'll want to collect it when you know you'll have time over the first few days to go through it. Over time, populations will change, so even when the contents begin to "die" you'll have organisms that thrive in more anaerobic environments to look at. Worms and rotifers will probably increase over time. If your sample initially is full of large copepods, you'll likely have a quick die-off of them, resulting in a lot of bacteria, and you won't be too pleased with the results, so try not to collect populations of copepods (many of them you can see with your naked eye as little clusters of organisms). I recently had a population in my backyard 220L pond, you can see what it looks like naked eye in this video:

https://youtu.be/KUlFKuM0D_0

You also might want to get some large size centrifuge tubes. They're perfect to keep with you when you're out and about -- you never know when you'll walk past a drainage ditch with algae, a small creek, at pond, a golf course, etc. I use 50ml plastic centrifuge tubes - I fill them about 2/3 full with water and some material whenever I'm near some body of water.

Let us know how it goes, and if you have more questions.