The videos make it look easy but sometimes it can be a challenge to do it correctly. Purging air from your gauges is often forgotten but can make a big difference in system performance when you're done. Even a small amount of air in the system will cause higher than normal pressures on the high side. Remember to purge air from your gauge lines. Also, proper removal of the gauges from the system so you don't take a line full of liquid refrigerant with you after charging, is important. Doing that improperly can leave you 1-2 oz low right off the bat.
Other challenges when charging are that the cans will often freeze up and stop going into the system. You add a bit of weight to the can by the condensation, as well. Simple solution is to drop the can into a bucket of hot water but then it becomes nearly impossible to weigh in. What I did (before getting an automated machine) was to weigh in an initial small amount, then put in full cans after that. The initial charge goes in the easiest so that's easiest to weigh. For example, if the system holds 32 oz, I'd put in 8 oz from the first can, set that can aside and then put in two full cans after that, using my hot water bucket.
Note that even trying to weigh in an initial amount like 8 oz can be challenging from a small can. If the system is not running you can charge as a liquid (can upside down) but don't do that with the AC running or you'll damage the compressor. Always charge as a vapor (can right side up) into the low side if the AC is running during charge. You see so many people shaking the cans, tipping them over and back, etc. If the system is running, charge via low side as vapor only. I often was able to put in the full charge, via liquid through the high side, with the system off, using a bucket of hot water and dropping the can in that.
It's still not super accurate but was the best I could do. If you don't do this all the time it's often just not worth the trouble, the hassle, and the possibility of damage to the system due to doing something wrong. It's usually $100-150 to have it done and the machines are extremely accurate, even compensating for the refrigerant in the hoses.