how to catch the fololowing pokemon

With the exception of Feebas, you must have obtained the National PokeDex to be able to encounter or obtain these pokemon. Keep that in mind.

23. Ekans - Insert Pokemon Fire Red into the GBA slot on your DS, but don't play the game. On Platinum, head to the southern area of Route 212. Ekans will now be found there, although encounters are uncommon.

27. Sandshrew - Insert Pokemon Leaf Green into the GBA slot on your DS, but don't play the game. On Platinum, head to Wayward Cave. Sandshrew will now be found there, although encounters will be uncommon.

37. Vulpix - Insert Pokemon Leaf Green into the GBA slot on your DS, but don't play the game. On Platinum, head to Route 209. Vulpix will now be found there, although encounters will be uncommon.

46. Paras - A daily binocular Pokemon found in the Great Marsh. Encounters are uncommon.

58. Growlithe - Insert Pokemon Fire Red into the GBA slot of your DS, but don't play the game. On Platinum, Growlithe will now be found on Routes 201 and 202, although encounters will be uncommon.

109. Koffing - Found in the interior of Stark Mountain, but it's rare.

120. Staryu - Fish with the Super Rod in Canalave City or Sunyshore City. It's rare.

131. Lapras - Victory Road (Surf in the cave that exits to Route 224).

165. Ledyba - Trade from HeartGold/SoulSilver, transfer from the GBA games or breed Ledian. They cannot be found otherwise.

331. Cacnea - Found on Route 228, but rarely encountered.

349. Feebas - Found in the Fog Room in Mt. Coronet (the along the path that takes you to Snowpoint). It's ultra rare, but you can fish for it with any Rod. The Super Rod is probably the most recommended. I don't think you need the National PokeDex for this one, but I'm not 100% sure.

Note that all of these Pokemon, bar Cacnea and Feebas, are easily found in HeartGold and/or SoulSilver. If you have those games and a spare DS, you're sorted.

Ekans- Put Pokemon FireRed into slot 2 and Pokemon Platinum into slot 1. Play Pokemon Platinum. Ekans will now be found at Route 212.
Sandshrew- Put Pokemon LeafGreen into slot 2 and Pokemon Platinum into slot 1. Play Pokemon Platinum.
Sandshrew will now be found at Wayward Cave.
Vulpix- Put Pokemon LeafGreen into slot 2 and Pokemon Platinum in slot 1. Play Pokemon Platinum. Vulpix will now be found at Route 209.
Paras- Found in the Great Marsh.
Growlithe- Put Pokemon FireRed into slot 2 and Pokemon Platinum into slot 1. Play Pokemon Platinum. Growlithe will now be found at Route 201 and at Route 202.
Koffing- Found at Stark Mountain and at Route 227.
Staryu- Found at Canalave City and at Sunyshore City while fishing with a super rod.
Lapras- Found at Victory Road.
Ledyba- Trade a Ledyba from Heart Gold or from Soul Silver, or breed Ledian.
Cacnea- Found at Route 228.
Feebas- Found at Mt. Coronet while fishing with a rod in fog.

They can all be traded from Heart Gold or from Soul Silver.