The short answer for a hardy but warm-water fish
Guppies travel best in doubled fish bags holding one-third of their home tank water and two-thirds air, packed upright inside an insulated box. Guppies have a reputation for toughness, and they do forgive more than many tropical fish, but a long journey still tests them in two specific ways. Their water must stay warm, because a small bag in a cool car loses heat fast. It must also stay clean, because guppies are eager eaters whose waste builds quickly in an unfiltered bag. Warmth, clean water, and sensible grouping carry this species through.
Grouping guppies before you bag them
Deciding who shares a bag is the guppy-specific step owners most often skip. Long-finned males travel comfortably together in modest numbers, but cramming a whole colony into one bag crowds the water and invites nipped tails by journey's end. A heavily pregnant female deserves a roomy bag to herself, since jostling and stress on a long trip can prompt her to drop fry early into water that cannot support them. Fast every fish for a day before departure, and catch them with a cup or soft net in calm water so flowing fins are not torn in a frantic chase around plants and ornaments.
Packing bags to hold tropical warmth
Water from the guppies' own tank fills each bag about one-third full, with the largest practical air pocket tied in above it. Double every bag, with the second tied from the opposite end, and stand them upright in the insulated box wedged gently so they cannot fall sideways. The insulation does the real temperature work on a long journey, slowing heat loss hour by hour. Position the box where cabin warmth reaches it without sitting it against a hot vent, and in cold weather add a safely wrapped warm pack outside the bags, never touching them directly, so no single bag cooks while others chill.
Looking after guppies while travelling
A sealed, dark box is the right travelling environment for guppies, and it should stay sealed. Each peek drops warmth, floods the fish with light, and starts them swimming hard in water you cannot change. Feed nothing on the road, however long the day runs, because uneaten food and fresh waste are the fastest route to foul bag water. Keep the box level on a seat or footwell rather than in a cold trunk, and carry it indoors with you on long stops instead of leaving guppies in a parked car whose temperature will drift. Smooth driving genuinely helps, since constant sloshing exhausts small fish.
Arrival, release, and watching the group
Floating each sealed bag in the receiving tank brings guppy water and tank water to the same temperature gradually. Add small amounts of tank water to the open bag in stages before releasing the fish, and give long-finned males and any pregnant female the gentlest handling of all. Dim the lights, let the group find the plants, and offer a light feed once they are swimming and begging normally. Check over the following days for clamped fins, shimmying in place, or white dusting on tails, and get advice from an aquatic veterinarian if those signs spread through the group. Keeping each guppy group's water, feeding, and fry notes in a KinsLe Pet Passport spares the receiver from guessing at the conditions they left.