Nakon posla perem ruke sapunom i stavljam kremu na kožu.

Because nakon (after) requires the genitive case.
So posao (nominative) changes to posla (genitive):

• posao → posla = after work
  This is a very common pattern: nakon + Genitive.

Both relate to washing, but they differ in aspect:

• perem (imperfective) = I wash / I am washing (process, habit, repeated action)
• operem (perfective) = I wash (and finish), often closer to I wash up / I get them washed (completely)

In a routine sentence like this (what I do after work), imperfective (perem) is very natural.

Ruke is accusative plural (direct object) of ruka (hand).
You’re washing what? → ruke.

Singular vs plural:

• perem ruke = I wash my hands (normal in English too)
• perem ruku would be I wash a hand / I’m washing one hand (less typical unless you mean one hand specifically).

Sapunom is instrumental singular of sapun (soap).
Instrumental often expresses means/tool: with what? → sapunom (with soap).

So:

• sapun (basic form)
• sapunom (instrumental) = with soap

In this context it’s clearly with soap (means).
Instrumental can also mark company (with someone), but then Croatian typically uses s + instrumental for clarity, especially with people:

• s prijateljem = with a friend
  For tools/means, plain instrumental without s is very common:
• pišem olovkom = I write with a pencil
• perem ruke sapunom = I wash my hands with soap

Kremu is accusative singular of krema (cream), because it’s the direct object of stavljam (I put/apply).

• krema (nominative)
• kremu (accusative) = cream as the thing being put/applied

Because na changes meaning depending on the case:

• na + accusative = movement/putting onto a surface (onto) → na kožu
• na + locative = location (on) → na koži

Here you are putting cream (a change of state / directed action), so Croatian uses accusative:

• stavljam kremu na kožu = I put/apply cream onto the skin

If describing where something already is:

• Krema je na koži. = The cream is on the skin.

By default, koža here is understood as your own skin from context, even without moja (my). Croatian often omits possessives when it’s obvious, especially with body parts and routine actions.
If you really want to specify, you can say:

• stavljam kremu na svoju kožu = I put cream on my (own) skin (more explicit/emphatic)

It’s flexible, and your sentence is already very natural. Common alternatives include:

• Nakon posla perem ruke sapunom i stavljam kremu na kožu. (neutral)
• Perem ruke sapunom i nakon posla stavljam kremu na kožu. (shifts focus)
• Nakon posla ruke perem sapunom… (more emphasis on ruke, but can sound marked)

Croatian uses word order more for focus/emphasis than for basic grammar, because cases show roles.

They can mean both; context decides:

• With Nakon posla (After work), it strongly suggests a habit/routine: After work, I wash… and apply…
• If you add “right now” context, present can be ongoing:
  • Sad perem ruke. = I’m washing my hands now.