Launch & Inflation
There are two prevailing methods for filling and attaching the balloon. Each method shares some elements but diverge part-way through preparations. A certain amount of side preparation and similar precautions must be taken in either case.
Regardless of the filling method you use below, you will need to have a way to measure the neck lift (total lift less the weight of the balloon), which is lift that your payload train will “see” and the lift that you calculated before. You can use a fish scale to measure the lift or you can attach a bag full of pre-weighted items that have the same weight as the amount of lift you want. In a pinch, you could even fill a water jug with the right amount of water to measure the lift. When the balloon and weight combined are neutrally buoyant then you have achieved your lift goal. Less lift will cause your balloon to ride slower but go higher and probably further away too. The converse is true if you give your balloon more lift.
If some outside air leaks into the balloon you will still be able to do your mission but this will lower the maximum altitude of your flight. If you dare to use Hydrogen as the lifting gas instead of Helium then the leaked air will also greatly increase the danger from an accidental ignition: the Hydrogen will burn rapidly in either case, but when it is already mixed with air (containing Oxygen) the explosion will be even more severe. The greatest danger from a good fill when using Hydrogen is if burning latex gets onto a member of the launch crew, but when outside air is mixed in too the extra strong detonation can make things much worse.
Any oils, from your hair, hands, or otherwise will damage your balloon so you will want to wear a soft cap and gloves when filling the balloon. Even a soft tree bud can be enough to pop a balloon so much sure you have plenty of open space to launch and a clear area to fill the balloon. The balloon can be inflated from atop a tarp on the ground as long as the tarp is reasonably clean.
You can buy a nice pre-made filling device, or cobble together your own with a pressure regulator, PVC pipe, a valve, PTFE tape (e.g. Teflon™), and a few connectors. , Zip ties will help keep the balloon attached with a mostly-airtight seal, but make sure you have a way to remove the zip ties when you are done.
Once the balloon is filled and removed from the filler then you will need to create a secure loop with the neck of the balloon. To say that this is a pseudo-science would be giving this part of the process too much credit, but it does work. Through some combination of rubber bands and zip ties the neck of the balloon is closed off, the neck is folded in half, and the neck is secured in place to make a loop of the load line to run through.
The other popular way to fill the balloon and attach it to the load line is to create a device that can do both. The idea is to get a PVC pipe the fits nicely into the neck of the balloon that also has a fitting on the bottom the fit the gas line connector. The filler is inserted and secured in the balloon with zip ties well before the launch. The pipe can have grooves machined into it to give the zip ties extra “bite” when holding the balloon in place. While securing the filler you can also connect cord for later attachment to the payload train or bottom of the filler can have places to attach the cord.
The way you recover your payload is largely dependent on the systems you decide to use to track it. The most important thing to note is that you must get permission from property owners to go onto their land.