Unless you have someone to keep your baby 100% occupied, I would really recommend doing this while your baby is asleep. Mostly because of the boiling water, but also because you need to keep your eye on the process and you don't want to get distracted and burn your house down.
Hemp is a natural fabric, which means that when it's brand new it is coated with natural oils. You need to strip these oils off before using the diaper, or it won't absorb much. You can do this by washing in hot water in your washing machine and drying 4-6 times, or by boiling on the stove. Boiling uses a lot less water, especially for something small like inserts. You don't want to boil anything with PUL, snaps, or elastic because that will ruin the diaper, but pocket inserts and prefolds are generally ok to boil.
Today, I'm prepping 4 new Thirsties hemp inserts. These are the best pocket inserts that I've found. They're incredibly trim, but hold a ton of pee. Arthur can be a bit of a super soaker, but a single Thirsties insert will hold him for at least 2 hours. The first thing that you'll notice is that they're huge.
On the right is one of the inserts that is already prepped and heavily used. When fabrics are prepped, they shrink. Once these inserts are done being prepped, they'll be about the same size as the one on the right.
Get a pot of water boiling. Use a stock pot if you have one, especially if you're boiling prefolds. I'm only doing 2 inserts at a time so a regular pot will do.
Ugh, that's sideways. Sorry! Add the inserts to the pot, set the timer for 15 minutes. Stick close by the stove, and stir the inserts around. If you want you can add a squirt of dish soap to help draw the oils out, but I don't usually remember to do that.
See how the water is all gross and yellow-ish? That's the hemp oil that has been boiled out of the fabric. After your 15 minutes are up, if you're a competent person you will have a pair of tongs in your kitchen. Pick the diapers out of the pot with the tongs, put them in a bowl/bucket and bring them to your washing machine. I don't have tongs, so I just use potholders and dump the whole pot into the washer. Now you're going to run a hot wash with a little bit of detergent. I usually take this opportunity to wash some towels or sheets. Use whatever detergent you're using for your diapers (I use Gain on everything, to keep it simple) and run a normal hot wash. Put everything in the dryer when that's finished, and you're done! At this point they're ready to go and should be close to full absorbency, but I personally put them straight from the dryer into the wetbag and wash them again with the next load of diapers. Pretty simple!
Like I said, you can do this with prefolds too, but you are going to want to use a big pot. Before my son was born I bought 18 brand new prefolds and I had plans to boil them because I didn't have hot water in my washing machine. I kind of procrastinated, and then my son was born early. Then I found myself with a 4 day old baby, boiling 2 prefolds at a time on the stove. I don't recommend it. If possible, do any prepping and boiling before your baby arrives.