"And we have seen and testify that the Father has sent the Son as Savior of the world." (1 John 4:14)
There are 3 ways you testify. One of them is by declaring before the Lord Himself what He did for you. You count your blessings and testify of His goodness in your personal life. You're conscious of, and thankful for, all He's done for you, in you, and with you. You affirm that all these things happened by His grace, mercy, faithfulness, and love that endure forever.
Secondly, you testify with your offering, and this is the part that not many understand and practice: the power of the offering. It's a testimony! When you give God your offering, it's not a donation, it's a hallowed oblation with which you're testifying that He's God and not a man. With your offering, you're testifying and giving glory to Him in your life.
The Patriarchs-- Abraham, Isaac, Jacob, David, and his son Solomon-- all gave offerings upon the altar in worship to God. Take Solomon, for example:
(1 Kings 3:4)
"...Solomon offered a thousand burnt offerings on that altar."
Solomon offered a thousand burnt offerings where God would have accepted a much lesser number; but it reflected His recognition and testimony of the greatness of God.
Apart from your offering being a celebration of the prosperity of God in your life, it's also your affirmation or testimony of His greatness, and your recognition of His power and sovereignty.
The third way to testify is to tell others what the Lord has done in, with, and for you. You tell of His goodness before men, living every day in faith, bearing fruits of righteousness. You're always joyful, no matter the circumstances.
It's a testimony of faith and confidence in the Lord when people see you and observe that you're joyful and unruffled in the face of diverse tests. They see that, regardless of the pressures and adversaries you face, you're steadfast, unmoved, and always abounding in the work of the Lord; therefore, they're inspired by your life of faith and trust in the Lord. That's a testimony!
These are basic ways that you can testify to the glory of God in your life. Hallelujah!
Blessed Father, You're great, and greatly to be praised. Thank You for the blessings and miracles I experience in my life, which are acts of Your love and kindness. The heavens and earth declare Your mighty works, and testify of Your infinite wisdom and grace, even as all creation sings "Hallelujah" to You, the eternal King of glory. Blessed are You forever. Amen.
Further Study: Luke 17:17-19; Revelation 12:11