Suppose an asset is brought

Suppose an asset is brought with an initial cost of $300. Let’s say the depreciation method is STL with 3 years life and zero residual value

1st year when you brought the asset
Asset Cost A/C Dr  300
                       Cash A/C Cr 300
At the end of first year, you ‘ll expense  $100
So
Depreciation Expense A/C Dr 100
                          Acc Depreciation A/C Cr 100

In BS
the asset value ‘ll remain as 300-100=200
In P&L, the expense ‘ll reduce the net income.

In 2nd & 3rd year you ‘ll also depreciate by 100 thus the accounting entries ‘ll be like
Depreciation Expense A/C Dr 100
                          Acc Depreciation A/C Cr 100