The Westminster Larger Catechism provides excellent instruction to those who participate in the Lord's Supper. Before coming to the supper, contemplate the truths below.
Q. 168. What is the Lord’s supper?
A. The Lord’s supper is a sacrament of the New Testament, wherein, by giving and receiving bread and wine according to the appointment of Jesus Christ, his death is showed forth; and they that worthily communicate feed upon his body and blood, to their spiritual nourishment and growth in grace; have their union and communion with him confirmed; testify and renew their thankfulness and engagement to God, and their mutual love and fellowship each with the other, as members of the same mystical body.
Q. 169. How hath Christ appointed bread and wine to be given and received in the sacrament of the Lord’s supper?
A. Christ hath appointed the ministers of his Word, in the administration of this sacrament of the Lord’s supper, to set apart the bread and wine from common use, by the word of institution, thanksgiving, and prayer; to take and break the bread, and to give both the bread and the wine to the communicants: who are, by the same appointment, to take and eat the bread, and to drink the wine, in thankful remembrance that the body of Christ was broken and given, and his blood shed, for them.
Q. 170. How do they that worthily communicate in the Lord’s supper feed upon the body and blood of Christ therein?
A. As the body and blood of Christ are not corporally or carnally present in, with, or under the bread and wine in the Lord’s supper, and yet are spiritually present to the faith of the receiver, no less truly and really than the elements themselves are to their outward senses; so they that worthily communicate in the sacrament of the Lord’s supper, do therein feed upon the body and blood of Christ, not after a corporal and carnal, but in a spiritual manner; yet truly and really, while by faith they receive and apply unto themselves Christ crucified, and all the benefits of his death.
Q. 171. How are they that receive the sacrament of the Lord’s supper to prepare themselves before they come unto it?
A. They that receive the sacrament of the Lord’s supper are, before they come, to prepare themselves thereunto, by examining themselves of their being in Christ, of their sins and wants; of the truth and measure of their knowledge, faith, repentance; love to God and the brethren, charity to all men, forgiving those that have done them wrong; of their desires after Christ, and of their new obedience; and by renewing the exercise of these graces,by serious meditation, and fervent prayer.
Q. 172. May one who doubteth of his being in Christ, or of his due preparation, come to the Lord’s supper?
A. One who doubteth of his being in Christ, or of his due preparation to the sacrament of the Lord’s supper, may have true interest in Christ, though he be not yet assured thereof; and in God’s account hath it, if he be duly affected with the apprehension of the want of it, and unfeignedly desires to be found in Christ, and to depart from iniquity: in which case (because promises are made, and this sacrament is appointed, for the relief even of weak and doubting Christians) he is to bewail his unbelief, and labor to have his doubts resolved; and, so doing, he may and ought to come to the Lord’s supper, that he may be further strengthened.
Q. 173. May any who profess the faith, and desire to come to the Lord’s supper, be kept from it?
A. Such as are found to be ignorant or scandalous, notwithstanding their profession of the faith, and desire to come to the Lord’s supper, may and ought to be kept from that sacrament, by the power which Christ hath left in his church, until they receive instruction, and manifest their reformation.
Q. 174. What is required of them that receive the sacrament of the Lord’s supper in the time of the administration of it?
A. It is required of them that receive the sacrament of the Lord’s supper, that, during the time of the administration of it, with all holy reverence and attention they wait upon God in that ordinance, diligently observe the sacramental elements and actions, heedfully discern the Lord’s body, and affectionately meditate on his death and sufferings, and thereby stir up themselves to a vigorous exercise of their graces; in judging themselves, and sorrowing for sin; in earnest hungering and thirsting after Christ, feeding on him by faith, receiving of his fullness, trusting in his merits, rejoicing in his love, giving thanks for his grace; in renewing of their covenant with God, and love to all the saints.
Q. 175. What is the duty of Christians, after they have received the sacrament of the Lord’s supper?
A. The duty of Christians, after they have received the sacrament of the Lord’s supper, is seriously to consider how they have behaved themselves therein, and with what success; if they find quickening and comfort, to bless God for it, beg the continuance of it, watch against relapses, fulfill their vows, and encourage themselves to a frequent attendance on that ordinance: but if they find no present benefit, more exactly to review their preparation to, and carriage at, the sacrament; in both which, if they can approve themselves to God and their own consciences, they are to wait for the fruit of it in due time: but, if they see they have failed in either, they are to be humbled, and to attend upon it afterwards with more care and diligence.
The following two resources are provide additional thoughts on preparation for the supper.
Preparing for Communion - pg. 1