Although gundam models (non resin etc) require no glue to assemble, it's a good idea to connect the pieces securely so that it does not move even when it's connected. If you don't secure the pieces, there's a likely chance that it will move after you have sanded down the seam line and you'll have to file down even more. Make sure you have checked that everything is correct before you even think about reaching for the poly cement. Double check then triple check. You should test that both parts are the correct ones by connecting them gently. If the parts require a poly cap, and it seems like they don’t want to go in, use a part of a runner to ‘break’ the part in.

After you have confirmed that everything is in correct order, apply ‘some’ poly cement on the edges of the part which will make contact. When I mean ‘some’ I mean a tiny amount, otherwise when you push the two pieces together, the excess amount will just pour out from the sides.

When the pieces are connected, squeeze tightly. Use appropriate holding equipment (washing pegs, large paper clips, rubber band etc) to hold the parts together so they are nice and snug until the plastic dries.