How do I calculate an amortization schedule?

Your interest is 7% PER YEAR. Below is the table for each month for $20,000.00. This calculates your monthly payment to be $340.98.
If that is incorrect, then the table is wrong.

PRINCIPAL INTEREST BALANCE
1 224.31 116.67 19,775.69
2 225.62 115.36 19,550.06
3 226.94 114.04 19,323.13
4 228.26 112.72 19,094.86
5 229.59 111.39 18,865.27
6 230.93 110.05 18,634.34
7 232.28 108.70 18,402.06
8 233.63 107.35 18,168.42
9 235.00 105.98 17,933.43
10 236.37 104.61 17,697.06
11 237.75 103.23 17,459.31
12 239.13 101.85 17,220.18
13 240.53 100.45 16,979.65
14 241.93 99.05 16,737.71
15 243.34 97.64 16,494.37
16 244.76 96.22 16,249.61
17 246.19 94.79 16,003.42
18 247.63 93.35 15,755.79
19 249.07 91.91 15,506.72
20 250.52 90.46 15,256.20
21 251.99 88.99 15,004.21
22 253.46 87.52 14,750.75
23 254.93 86.05 14,495.82
24 256.42 84.56 14,239.40
25 257.92 83.06 13,981.48
26 259.42 81.56 13,722.06
27 260.93 80.05 13,461.13
28 262.46 78.52 13,198.67
29 263.99 76.99 12,934.68
30 265.53 75.45 12,669.15
31 267.08 73.90 12,402.08
32 268.63 72.35 12,133.44
33 270.20 70.78 11,863.24
34 271.78 69.20 11,591.46
35 273.36 67.62 11,318.10
36 274.96 66.02 11,043.14
37 276.56 64.42 10,766.58
38 278.18 62.81 10,488.40
39 279.80 61.18 10,208.61
40 281.43 59.55 9,927.18
41 283.07 57.91 9,644.10
42 284.72 56.26 9,359.38
43 286.38 54.60 9,073.00
44 288.05 52.93 8,784.94
45 289.73 51.25 8,495.21
46 291.42 49.56 8,203.78
47 293.12 47.86 7,910.66
48 294.83 46.15 7,615.83
49 296.55 44.43 7,319.27
50 298.28 42.70 7,020.99
51 300.02 40.96 6,720.96
52 301.77 39.21 6,419.19
53 303.53 37.45 6,115.65
54 305.31 35.67 5,810.35
55 307.09 33.89 5,503.26
56 308.88 32.10 5,194.38
57 310.68 30.30 4,883.70
58 312.49 28.49 4,571.21
59 314.31 26.67 4,256.90
60 316.15 24.83 3,940.75
61 317.99 22.99 3,622.76
62 319.85 21.13 3,302.91
63 321.71 19.27 2,981.20
64 323.59 17.39 2,657.61
65 325.48 15.50 2,332.13
66 327.38 13.60 2,004.75
67 329.29 11.69 1,675.47
68 331.21 9.77 1,344.26
69 333.14 7.84 1,011.12
70 335.08 5.90 676.04
71 337.04 3.94 339.00
72 339.00 1.98 0.00

Total…….2,628.00 20,000.00