Guatemala and Costa Rica are great sail fishing locations.
Most of the fishing done down there is trolling, so there aren't a lot of "tips" to give. Your boat will set out lines, you'll troll through likely areas, and if a sail hits, you'll grab the rod and hold on.
In the States, some places will fish for sails by freelining live bait, and that takes a little more skill and involvement from the angler, but I've never been on a trip "south of the border" where they did anything other than trolling.
If you're lucky, they'll catch some ballyhoo or other forage fish and troll a live bait or two. Live baits will usually out-perform artificials, but you pretty much take what you can get on the charter trips.
If you are prone to sea sickness, take a dramamine well in advance of going out. You'll be offshore, most of the fishing done down there is out of small boats, and if it's rough, you will be miserable if you're not prepared.
Book a full day rather than a half day if you can afford it. Offshore trolling involves covering a lot of water, and the more time you can spend, the better your odds will be.
Don't forget your sunscreen either. The sun is INTENSE down that close to the equator, and you will fry in an instant on the open ocean.
Going in June, you are going to be off the peak season for sails in Guatemala. It's really probably better a lot earlier in the year. Fishing can still be good, but check and make sure it's worth targeting sailfish.
If the sails aren't happening, you can always ask about targeting dorado (mahi mahi) or fishing closer inshore for big roosterfish instead. Either one can still make for a very fun day of fishing.
bd