Never done pants filling before, but thinking about it. What type of pants does one normally get for this? I supposed it needs to be somewhat tight to keep the mess in?

You're right, tight -- but not too tight, or you won't get much in!

Boxer shorts (but not too loose) worn in reverse would be a good tip

Pants as in underpants, or pants as in trousers?

Tracksuit trousers worn over sports shorts is a nice "filling" outfit, or wear it reversed, the trackies (with no underwear) against the skin and then a pair of sports shorts over the top, fill the shorts, feel the mess soak through the trackies and watch it flow out of the shorts and over the outside of the trousers too.

Women's one-piece swimsuits are also great fun to fill, and because they are supported over the shoulders there's no risk of them sagging, no matter how much gloop you pour in.

Nice tight boxers are great for filling. You may find some goop goes down the legs but if they're tight enough they'll keep everything where you want it.

Otherwise briefs or speedos are excellent for filling.

Just try out different items and see what works best for you!

I'd say briefs with 501s worn over them. Stuff 'em good and then button up the jeans to keep it all in.

Loose fitting trousers with suspenders and traditional boxer shorts underwear. A full pie fits well both front and back that way. Lots of room for the mess and ooze to run down the legs.

Form-fitting underwear and tight jeans are a good starting point. +1 to the individual who recommended tracksuit pants, they can hold a lot and feel great when the mess is pooled!

PVC panties are pretty awesome for this.

In general you'll want full size pants- no thongs or strings, and tight fitting around the legs if you want to hold the mess in there.

What's also a lot of fun is dunking and soaking some knickers in a bucket of gunge, and scooping some extra handfuls in there before slipping them on.
Or defrosting a large and sticky gateaux/creamy cake in them, then slipping them on and gently pulling them up to keep the cake intact ... before you sit down and/or receive a wedgie.

Have fun

My favorite combination is briefs (or mid to high rise bikini panties) and sweatpants. Filling with lotion and shaving cream is the best!

Always boxer briefs, especially fun patterns or colours!

Men's competitive swimsuits are amazing for filling.

Leggings work well. Not to tight and hold a lot of whatever you put in them.

I swear by all the gods.:: the tighter is not the better for filling! It's all about the waist and ankles!

I found that OLD SCHOOL WINDBREAKERS! Like the sets from the 90s! they are the best! I used them for holding water and whoa! I was so shocked! Just check out the ankle part and see what kind they have and if it's not a good sturdy ankle? You could pair with some welliez? Then duct tape! Ha!

If you have 501s with a 24 inch waist? I'll buy them from you! MY FAVE!

Wetsuit pants or shorts are very good for filling

Ok I'm really confused here, because lots of recommendation for shorts, but I thought it needed to be long pants (trousers) so that you can keep the slime in there?

I'm such a noob, but what would you buy exactly? Would you get something like this and just buy a size smaller than normal: https://www.amazon.com/Hevto-Wetsuits-Neoprene-Surfing-Sports/dp/B09K4Z5LSC/

They'll do you fine, and no need to go a size smaller - just get your usual size.

If you're worried about them slipping down with the weight of all the mess, buy some braces/suspenders too to hold them up.

Tight elastic is best so as to keep all that lovely goo inside. Anything see thru when wet is also a fav, so: Plastic, white, pail colours as good. For me, everyday classic bikini,s are good, more so if you have to ware it around the place as well ie: Party's, walking the pooch, car park or where ever else. In terms of sloppy stuff, raw eggs with shells do it best, there cold, can crunch, slimy and otherwise so wonderfully disgusting that its make raw eggs the best ever. ... there
Oh, PS, Please do send me pics of your raw eggs smashed in the undie-pants, would love to have a peep, please